Port to FreeBSD.

Use FreeBSD-specific functions (_pthread_mutex_init_calloc_cb(),
_malloc_{pre,post}fork()) to avoid bootstrapping issues due to
allocation in libc and libthr.

Add malloc_strtoumax() and use it instead of strtoul().  Disable
validation code in malloc_vsnprintf() and malloc_strtoumax() until
jemalloc is initialized.  This is necessary because locale
initialization causes allocation for both vsnprintf() and strtoumax().

Force the lazy-lock feature on in order to avoid pthread_self(),
because it causes allocation.

Use syscall(SYS_write, ...) rather than write(...), because libthr wraps
write() and causes allocation.  Without this workaround, it would not be
possible to print error messages in malloc_conf_init() without
substantially reworking bootstrapping.

Fix choose_arena_hard() to look at how many threads are assigned to the
candidate choice, rather than checking whether the arena is
uninitialized.  This bug potentially caused more arenas to be
initialized than necessary.