maint: use "const" and "pure" function attributes where possible

maint: use "const" and "pure" function attributes where possible

* configure.ac (WARN_CFLAGS): Add -Wsuggest-attribute=const,
-Wsuggest-attribute=pure and -Wsuggest-attribute=noreturn.
(GNULIB_WARN_CFLAGS): But do not add them here... yet.
* src/chown-core.h (chopt_free, uid_to_name): Add function attribute(s).
* src/copy.c (is_ancestor, valid_options): Likewise.
* src/copy.h (chown_failure_ok): Likewise.
* src/dd.c (operand_matches, operand_is): Likewise.
* src/df.c (selected_fstype, excluded_fstype): Likewise.
* src/expr.c (null looks_like_integer): Likewise.
* src/md5sum.c (hex_digits): Likewise.
* src/od.c (get_lcm): Likewise.
* src/pathchk.c (component_start, component_len): Likewise.
* src/pinky.c (count_ampersands): Likewise.
* src/pr.c (cols_ready_to_print): Likewise.
* src/ptx.c (search_table): Likewise.
* src/sort.c (find_unit_order): Likewise.
* src/stty.c (mode_type_flag, string_to_baud, baud_to_value): Likewise.
* src/system.h (gcd, lcm): Likewise.
* src/tr.c (is_char_class_member, look_up_char_class): Likewise.
(star_digits_closebracket): Likewise.
* src/uniq.c (find_field): Likewise.
* src/wc.c (compute_number_width): Likewise.
* lib/xfts.h (cycle_warning_required): Likewise.
* gl/lib/randint.h (randint_get_source): Likewise.
* gl/lib/randperm.c (ceil_lg): Likewise.
* gl/lib/randperm.h (randperm_bound): Likewise.
* lib/strnumcmp.h (strintcmp): Likewise.