Apply new CentOS fix for strdup null-test bug (re: 7afb30e) (#255)

This is an update to one of Red Hat's patches. The strdup change is
from CentOS:
https://git.centos.org/rpms/ksh/blob/c8s/f/SOURCES/ksh-20120801-annocheck.patch

The reason why gcc (and also clang) optimize out the null check is
because the glibc string.h header gives 's' a nonnull attribute (in
other words, this is a glibc compatibility bug, not a compiler bug).
Clang gives the following informative warning when compiling strdup:

> /home/johno/GitRepos/KornShell/ksh/src/lib/libast/string/strdup.c:66:10: warning: nonnull parameter 's' will evaluate to 'true' on
>         return (s && (t = oldof(0, char, n = strlen(s) + 1, 0))) ? (char*)memcpy(t, s, n) : (char*)0;
>                 ^ ~~
> /usr/include/string.h:172:35: note: declared 'nonnull' here
>      __THROW __attribute_malloc__ __nonnull ((1));
>                                   ^
> /usr/include/sys/cdefs.h:303:44: note: expanded from macro '__nonnull'
> # define __nonnull(params) __attribute__ ((__nonnull__ params))

The proper fix is to rename the function in strdup.c to
'_ast_strdup'. This avoids the string.h conflict and fixes the Red
Hat bug. I've also made a similar change to getopt.c, since clang
was throwing a nonnull warning there as well.

src/lib/libast/features/map.c (which generates FEATURE/map which is
indirectly included by everything) is updated to always map getopt
to _ast_getopt and strdup to _ast_strdup.

src/lib/libast/comp/getopt.c

@@ -43,8 +43,12 @@ char*		optarg = 0;
 
 static int	lastoptind;
 
+/*
+ * Avoid a null-test optimization bug caused by glibc's headers
+ * by naming this function '_ast_getopt' instead of 'getopt'.
+ */
 extern int
-getopt(int argc, char* const* argv, const char* optstring)
+_ast_getopt(int argc, char* const* argv, const char* optstring)
 {
 	int	n;