alloc() never returns Null

Mark alloc()--also dupstr() and re_alloc()--for gcc and clang as
always returning non-Null.  This should silence some of the static
analysis complaints.

Almost all the monster and object naming functions (anything that
returns an mbuf or an obuf) should be marked this way too but I'll
leave that for somebody else to deal with.

I didn't attempt to mark alloc() with the 'malloc' attribute because
macro definitions could end up causing trouble.  Specifying its
deallocator would probably be useful but is at even bigger risk of
macro interference.

I'm not sure whether gcc 3 is really the right test for whether the
returns_nonnull attribute setting is available.

include/tradstdc.h

@@ -404,6 +404,7 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #if __GNUC__ >= 3
 #define UNUSED __attribute__((unused))
 #define NORETURN __attribute__((noreturn))
+#define NONNULL __attribute__((returns_nonnull))
 #if !defined(__linux__) || defined(GCC_URWARN)
 /* disable gcc's __attribute__((__warn_unused_result__)) since explicitly
    discarding the result by casting to (void) is not accepted as a 'use' */
@@ -422,5 +423,8 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #ifndef NORETURN
 #define NORETURN
 #endif
+#ifndef NONNULL
+#define NONNULL
+#endif
 
 #endif /* TRADSTDC_H */