Use intmax and uintmax to support 64-bit off_t, time_t, &c.

Use intmax and uintmax to support 64-bit off_t, time_t, &c.

Eliminate BIGNUM_NO_ULONG.  Twenty years ago it may have made sense;
now too much of the system relies on having unsigned longs for it to
matter.

New routines convert between integers and intmax/uintmax.  The old
ones for long/unsigned long are still there, because they are likely
to be much faster on 32-bit systems.  Only a few uses of long have
been replaced by intmax -- the ones where it was obvious we were
converting between time_t, off_t, ino_t, &c., and long.  Others may
still be there.  Before:

(file-length "fnord")
;Value: 1235

After:

(file-length "fnord")
;Value: 4294968531

(`fnord' was created by seeking to byte 2^32 + 1234 or something and
writing a single byte there.)