maint: improve static and dynamic checking

maint: improve static and dynamic checking

This modernizes the source code somewhat, to take advantage
of advances in GCC over the years, and Gnulib’s ‘assure’ module.
Include assure.h in files that now need it.
Do not include assert.h directly; it’s no longer needed.
* bootstrap.conf (gnulib_modules): Add ‘assure’.
* gl/lib/randread.c (randread_error):
* src/chmod.c (describe_change):
* src/chown-core.c (describe_change):
* src/cp.c (decode_preserve_arg):
* src/head.c (diagnose_copy_fd_failure):
* src/ls.c (parse_ls_color):
* src/od.c (decode_one_format):
* src/split.c (main):
* src/test.c (binary_operator, posixtest):
Prefer affirm to abort, since it has better diagnostics in the
normal case and better performance with -DNDEBUG.
* gl/lib/xdectoint.c, src/die.h: Include stddef.h, for unreachable.
* gl/lib/xdectoint.c: Do not include verify.h; no longer needed.
* gl/lib/xdectoint.c (__xnumtoint):
* src/die.h (die):
Prefer C23 unreachable () to assume (false).
* gl/lib/xfts.c (xfts_open):
* src/basenc.c (base32hex_encode):
* src/copy.c (abandon_move, copy_internal, valid_options):
* src/cut.c (cut_fields):
* src/df.c (alloc_field, decode_output_arg, get_dev):
* src/du.c (process_file, main):
* src/echo.c (usage):
* src/factor.c (udiv_qrnnd, mod2, gcd2_odd, factor_insert_large)
(mulredc2, factor_using_pollard_rho, isqrt2, div_smallq)
(factor_using_squfof):
* src/iopoll.c (iopoll_internal, fwrite_wait):
* src/join.c (add_field):
* src/ls.c (dev_ino_pop, main, gobble_file, sort_files):
* src/mv.c (do_move):
* src/od.c (decode_format_string, read_block, dump, main):
* src/remove.c (rm):
* src/rm.c (main):
* src/sort.c (stream_open):
* src/split.c (next_file_name, lines_chunk_split):
* src/stdbuf.c (main):
* src/stty.c (set_speed):
* src/tac-pipe.c (line_ptr_decrement, line_ptr_increment):
* src/touch.c (touch):
* src/tr.c (find_bracketed_repeat, get_next)
(validate_case_classes, get_spec_stats, string2_extend, main):
* src/tsort.c (search_item, tsort):
* src/wc.c (main):
Prefer affirm to assert, as it allows for better static
checking when compiling with -DNDEBUG.
* src/chown-core.c (change_file_owner):
* src/df.c (get_field_list):
* src/expr.c (printv, null, tostring, toarith, eval2):
* src/ls.c (time_type_to_statx, calc_req_mask, get_funky_string)
(print_long_format):
* src/numfmt.c (simple_strtod_fatal):
* src/od.c (decode_one_format):
* src/stty.c (mode_type_flag):
* src/tail.c (xlseek):
* src/tr.c (is_char_class_member, get_next, get_spec_stats)
(string2_extend):
Prefer unreachable () to abort () or assert (false) when merely
pacifying the compiler, e.g., in a switch statement on an enum
where all cases are covered.
* src/copy.c (valid_options): Now returns void; the bool was useless.
Caller no longer needs to assert.
* src/csplit.c (find_line):
* src/expand-common.c (next_file):
* src/shred.c (incname):
* src/sort.c (main):
* src/tr.c (append_normal_char, append_range, append_char_class)
(append_repeated_char, append_equiv_class):
* src/tsort.c (search_item):
Omit assert, since the hardware will check for us.
* src/df.c (header_mode): Now the enum type it should have been.
* src/du.c (process_file):
* src/ls.c (assert_matching_dev_ino):
* src/tail.c (valid_file_spec):
* src/tr.c (validate_case_classes):
Mark defns with MAYBE_UNUSED if they’re not used when -DNDEBUG.
* src/factor.c (prime_p, prime2_p, mp_prime_p): Now ATTRIBUTE_PURE.
Prefer affirm to error+abort.  No need to translate this diagnostic.
* src/fmt.c (get_paragraph):
* src/stty.c (display_changed, display_all, sane_mode):
* src/who.c (idle_string):
Prefer assume to assert, since the goal is merely pacification
and assert doesn’t pacify anyway if -DNDEBUG is used.
* src/join.c (decode_field_spec):
Omit unreachable abort.
* src/ls.c (assert_matching_dev_ino, main):
* src/tr.c (get_next):
Prefer assure to assert, since the check is relatively expensive
and won’t help static analysis.
* src/ls.c (main):
Prefer static_assert to assert of a constant expression.
(format_inode): Redo to make it clear that buflen doesn’t matter,
and that buf must have a certain number of bytes.  All callers changed.
This pacifies -Wformat-overflow.
* src/od.c (decode_one_format):
Omit an assert that tested for obviously undefined behavior,
as the compiler could optimize it away anyway.
* src/od.c (decode_one_format, decode_format_string):
Prefer ATTRIBUTE_NONNULL to runtime checking.
* src/stat.c: Do not include <stddef.h> since system.h does that now.
* src/sync.c (sync_arg):
Prefer unreachable () to assert (true), which was a typo.
* src/system.h: Include stddef.h, for unreachable.
* src/tail.c (xlseek): Simplify by relying on ‘error’ to exit.