Rename --warn-syntax to --warn-macro-sequence[=regex], to make it

Rename --warn-syntax to --warn-macro-sequence[=regex], to make it
more flexible, and so that autoconf can use it.
* src/m4.h (set_macro_sequence, free_macro_sequence): New
prototypes.
* src/builtin.c (macro_sequence_buf, macro_sequence_regs)
(macro_sequence_inuse, set_macro_sequence, free_macro_sequence):
New variables and functions.
(define_user_macro): Allow flexibility in regular expression used
to trigger warning.
* src/m4.c (warn_syntax): Delete.
(usage, WARN_MACRO_SEQUENCE_OPTION, main): Implement changed
spelling of option, along with optional argument.
* doc/m4.texinfo (Operation modes, Arguments): Document this
change.
* NEWS: Document this change.