Transition to C99

[JohnoKing]

This commit is a compilation of many old patches of mine. It removes obsolete code and enforces C99 as a build requirement.

Changes:

• Remove many unused tests and useless feature tests. (This was a bit more involved for <sys/types.h>, which had annoying special casing.) This included the "check for global headers" in the iffe script.
• Assume fchmod is present and use it instead of chmod in shcomp for very slightly better performance.
• dotpaths_lib(): Use fstat rather than stat for the PWD for better performance.
• Scrap pre-C99 fallbacks for iswblank(3).
• Replaced usage of the obsolescent timeofday() macro with the more portable tvgettime() function in libast, which itself uses the modern clock_gettime internally when it's available.
• Replaced usage of strcopy with either strcpy or stpcpy in cases where no buffer overlap occurs. There are perhaps more places where replacement is possible (ksh93v- and ksh2020 do this far more liberally), but I'm trying to be conservative with these changes to avoid accidentally introducing bugs.
• Replaced an instance of strncopy with strlcpy in keytrap() because the buffers don't overlap.
  • Also replaced some instances of strncpy with strlcpy (vide ksh2020 commit f63ebd0 and associated issue Replace uses of strncpy() with strlcpy() att/ast#266).
• Add pertinent documentation as it relates to str*copy's usage with overlapping buffers.
• Backported a modified version of ksh93v-'s stpcpy fallback.
• Introduced usage of C99 restrict, primarily for standards compliance in compatibility functions (though some regular functions like sfopen() will also use restrict now).
• Assume C99 __func__ is available and drop now unnecessary C90 feature tests.
• Added a few casts to fix some compiler warnings.
• Remove basic iffe probes for various C99 multibyte functions (excluding those that still need tests due to platform-specific bugs). Also removed other unnecessary probes for C99 and POSIX functionality.
• Remove unnecessary mktime.c, wc.c and wordexp.c compat files (the operating system should provide these functions).
• Switched to using C99-style struct init throughout the codebase.
• Removed usage of bzero, bcopy, memclear and memzero in favor of standard memset and memcpy.
• Minor bugfix: avoid passing overlapping buffers to getaddrinfo in io.c because that violates the POSIX standard.
• make.probe: Automatically use the latest C standard available (i.e. C23) because GCC/Clang may use something older by default whilst having support for newer standards.

I have additional changes for using C99-style for loops and bools, but those have been excluded because this patch is big enough as it is. I also encountered crashing bugs when I attempted to remove widespread usage of the struct hack, so the struct hack was only removed if it was already disabled by a preexisting __STDC_VERSION__ check.

Progresses #777 (this commit doesn't include or fix the removed _c99_in_the_wild code).