Replace JEMALLOC_OPTIONS with MALLOC_CONF.

Replace the single-character run-time flags with key/value pairs, which
can be set via the malloc_conf global, /etc/malloc.conf, and the
MALLOC_CONF environment variable.

Replace the JEMALLOC_PROF_PREFIX environment variable with the
"opt.prof_prefix" option.

Replace umax2s() with u2s().