c-stack: improve checking if !libsigsegv

If SIGINFO_WORKS, do not treat a null pointer dereference as if it
were a stack overflow.  Use uintptr_t and INT_ADD_WRAPV to avoid
unlikely pointer overflow.  Also, fix some obsolete code and typos.
I found these problems while looking into this bug report:
https://lists.gnu.org/r/grep-devel/2020-09/msg00053.html
* lib/c-stack.c: Include c-stack.h first, to test interface.
Include inttypes.h for UINTPTR_MAX, stdbool.h, stddef.h for
max_align_t, intprops.h for INT_ADD_WRAPV.
(USE_LIBSIGSEGV): New macro; use it to simplify later code.
(SIGSTKSZ): Simplify setup.  Work around libsigsegv bug only
for libsigsegv 2.8 and earlier since the bug should be fixed
after that.
(alternate_signal_stack): Use max_align_t instead of doing it by hand.
(segv_handler, overflow_handler, segv_handler) [DEBUG]:
Assume sprintf returns byte count; this assumption is safe now.
(page_size): New static volatile variable, since sysconf isn’t
documented to be async-signal-safe on Solaris.  This variable is
present and used if (!USE_LIBSIGSEGV && HAVE_SIGALTSTACK &&
HAVE_DECL_SIGALTSTACK && HAVE_STACK_OVERFLOW_HANDLING &&
SIGINFO_WORKS).
(segv_handler): Use it if present.  Never report null pointer
dereference as a stack overflow.  Check for (unlikely) unsigned
and/or pointer overflow.
* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC):
Rename cache variables to gl_cv_sys_stack_overflow_works
and gl_cv_sys_xsi_stack_overflow_heuristic.
All uses changed.
(gl_PREREQ_C_STACK): Do not require AC_FUNC_ALLOCA, since
c-stack no longer uses STACK_DIRECTION.
Do not check for unistd.h, since we depend on unistd.
Fix shell typo ‘$"ac_cv_sys_xsi_stack_overflow_heuristic"’.
* modules/c-stack (Depends-on): Sort.  Add intprops, inttypes,
stdbool, stddef.

modules/c-stack

@@ -7,15 +7,19 @@ lib/c-stack.c
 m4/c-stack.m4
 
 Depends-on:
-gettext-h
 errno
 exitfail
+getprogname
+gettext-h
 ignore-value
-unistd
+intprops
+inttypes
+libsigsegv
 raise
 sigaction
-libsigsegv
-getprogname
+stdbool
+stddef
+unistd
 
 configure.ac:
 gl_C_STACK