Add util unit tests, and fix discovered bugs.

Add unit tests for pow2_ceil(), malloc_strtoumax(), and
malloc_snprintf().

Fix numerous bugs in malloc_strotumax() error handling/reporting.  These
bugs could have caused application-visible issues for some seldom used
(0X... and 0... prefixes) or malformed MALLOC_CONF or mallctl() argument
strings, but otherwise they had no impact.

Fix numerous bugs in malloc_snprintf().  These bugs were not exercised
by existing malloc_*printf() calls, so they had no impact.