Renamed from strtoxmax.c, removing the old strtoimax.c.

Renamed from strtoxmax.c, removing the old strtoimax.c.

Also, make the following further changes to make this file's
configuration more similar to that of strtol.c:
(UNSIGNED): Renamed from STRTOUXMAX_UNSIGNED.  All uses changed.
(strtoumax, uintmax_t, strtoull, strtol): Remove.
(intmax_t, strtoimax, strtol, strtoll): New macros, if UNSIGNED.
(strtoimax): Renamed from strtoumax.  All uses of unsigned values
changed to signed values.

And make the following changes as well:
Fix copyright notice, as 1999 was missing.
(verify): New macro.
(strtoimax): Check sizes at compile-time, not run-time.
Prefer strtol to strtoll if both work.
(main): Remove; it was not that useful and was a pain to maintain.