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.

include/jemalloc/internal/util.h

@@ -85,7 +85,8 @@
 #ifdef JEMALLOC_H_EXTERNS
 
 int	buferror(int err, char *buf, size_t buflen);
-uintmax_t	malloc_strtoumax(const char *nptr, char **endptr, int base);
+uintmax_t	malloc_strtoumax(const char *restrict nptr,
+    char **restrict endptr, int base);
 void	malloc_write(const char *s);
 
 /*