Fix build on QNX

This makes ksh 93u+m build on the following system:

	$ uname -a
	QNX qnx 6.5.0 2010/07/09-14:44:03EDT x86pc x86

Thanks to polarhome.com for providing the QNX shell account.

There are a number of regressions left to work out:

arrays.sh[636]: copying a large array fails
bracket.sh[129]: /tmp/ksh93.shtests.1753215026.6923/bracket.C/original should be older than /tmp/ksh93.shtests.1753215026.6923/bracket.C/newer
bracket.sh[132]: /tmp/ksh93.shtests.1753215026.6923/bracket.C/newer should be newer than /tmp/ksh93.shtests.1753215026.6923/bracket.C/original
builtins.sh[683]: real_t1 not found after parent directory renamed in subshell
functions.sh[1023]: cannot handle comsub depth > 256 in function
io.sh[252]: <# not working for pipes
io.sh[337]: read -n3 from pipe not working
io.sh[346]: read -n3 from fifo failed -- expected 'a', got 'abc'
io.sh[349]: read -n1 from fifo failed -- expected 'b', got 'd'
io.sh[379]: should have timed out
io.sh[380]: line1 should be 'prompt1: '
io.sh[381]: line2 should be line2
io.sh[382]: line3 should be 'prompt2: '
io.sh[406]: LC_ALL=C read -n2 from pipe 'a bcd' failed -- expected 'a bcd', got 'ab cd'
io.sh[406]: LC_ALL=C.UTF-8 read -n2 from pipe 'a bcd' failed -- expected 'a bcd', got 'ab cd'
jobs.sh[86]: warning: skipping subshell job control test due to non-compliant 'ps'
pty.sh[105]: POSIX sh 026(C): line 120: expected "(Stopped|Suspended)", got EOF
pty.sh[128]: POSIX sh 028(C): line 143: expected "(Stopped|Suspended) \(SIGTTIN\)", got EOF
pty.sh[151]: POSIX sh 029(C): line 166: expected "(Stopped|Suspended) \(SIGTTOU\)", got EOF
signal.sh[310]: kill -TERM $$ failed, required termination by signal 'EXIT'
signal.sh[310]: kill -VTALRM $$ failed, required termination by signal 'EXIT'
signal.sh[310]: kill -PIPE $$ failed, required termination by signal 'EXIT'

(The io.sh failures mean libast sfpkrd() is not working.)

src/lib/libast/obsolete/spawn.c:
- Removed. Didn't compile due to wrong number of arguments to
  spawnve(2), but is obsolete and unused.

src/lib/libast/comp/localeconv.c:
- The initialisation of two static 'struct lconv' variables was
  done in a way that depended on OS headers declaring the struct
  members in a certain order. This holds on most systems, but not
  on QNX, and POSIX does not actually specify the order at all:
	https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/locale.h.html
  So each member must be initialised by name. But C89 does not
  support initialising struct members by name, so we have to do it
  using an initialiser function that simply assigns the values.

src/lib/libast/comp/spawnveg.c:
- Fix for systems without either P_DETACH or _P_DETACH.

src/lib/libast/features/vmalloc,
src/lib/libast/vmalloc/vmmopen.c,
src/lib/libast/Mamfile:
- Add test for sys/shm.h header. If it doesn't exist, as it doesn't
  on QNX, use the stub vmmapopen() as the real one won't compile.
  (Mamfile: Add dependency on FEATURE/vmalloc to vmmopen.c.)

src/lib/libast/vmalloc/malloc.c:
- Remove superfluous externs that are already provided by either
  AST or system headers. The 'void cfree' extern caused a build
  failure on QNX because cfree() is of type int on QNX.

src/lib/libast/comp/conf.tab:
- Remove check for _map_spawnve; src/lib/libast/RELEASE says it was
  removed.

src/lib/libast/vmalloc/malloc.c

@@ -998,36 +998,6 @@ extern Mstats_t mstats()
 
 #define setregmax(n)
 
-#undef	calloc
-extern Void_t*	calloc _ARG_((size_t, size_t));
-
-#undef	cfree
-extern void	cfree _ARG_((Void_t*));
-
-#undef	free
-extern void	free _ARG_((Void_t*));
-
-#undef	malloc
-extern Void_t*	malloc _ARG_((size_t));
-
-#if _lib_memalign
-#undef	memalign
-extern Void_t*	memalign _ARG_((size_t, size_t));
-#endif
-
-#if _lib_pvalloc
-#undef	pvalloc
-extern Void_t*	pvalloc _ARG_((size_t));
-#endif
-
-#undef	realloc
-extern Void_t*	realloc _ARG_((Void_t*, size_t));
-
-#if _lib_valloc
-#undef	valloc
-extern Void_t*	valloc _ARG_((size_t));
-#endif
-
 #if defined(__EXPORT__)
 #define extern		__EXPORT__
 #endif