Support fls().

The immediate impetus for this is that Noah Misch's patch to elide
unnecessary table and index rebuilds when changing typmod for temporal
types uses it; and this is extracted from that patch, with some
further commentary by me.  But it seems logically separate from the
remainder of the patch, so I'm committing it separately; this is not
the first time someone has wanted fls() in the backend and probably
won't be the last.

If we end up using this in more performance-critical spots it may be
worthwhile to add some architecture-specific optimizations to our
src/port version of fls() - e.g. any x86 platform can implement this
using the assembly instruction BSRL.  But performance won't matter
a bit for assessing typmod changes, so I'm not worried about that
right now.

src/include/port.h

@@ -396,6 +396,10 @@ extern double pg_erand48(unsigned short xseed[3]);
 extern long pg_lrand48(void);
 extern void pg_srand48(long seed);
 
+#ifndef HAVE_FLS
+extern int fls(int mask);
+#endif
+
 #ifndef HAVE_FSEEKO
 #define fseeko(a, b, c) fseek(a, b, c)
 #define ftello(a)		ftell(a)