libc/stdlib: Port strtonumx() from Illumos

Add strtonumx(), a companion to strtonum(3) that preserves its safety
and error-reporting semantics while allowing the caller to specify a
conversion base, similar to the strtol(3) family of functions.

Reviewed by: emaste, kib, ziaee
Obtained from: https://www.illumos.org/issues/15365
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54270