we now have a true replacement in strtoumax.c.
(__strtol): Always define to strtoumax.
(<stdlib.h>): No need to include.
(PARAMS): Remove.
(my_strtoumax): Move this to strtoumax.c,
rename it to strtoumax, and simplify.
whether overflow occurred. Improve overflow-detection to use
only one conditional branch total, rather than 2N+1
conditional branches for an N-digit number.