Resolve bootstrapping issues when embedded in FreeBSD libc.

b2c0d6322d (Add witness, a simple online
locking validator.) caused a broad propagation of tsd throughout the
internal API, but tsd_fetch() was designed to fail prior to tsd
bootstrapping.  Fix this by splitting tsd_t into non-nullable tsd_t and
nullable tsdn_t, and modifying all internal APIs that do not critically
rely on tsd to take nullable pointers.  Furthermore, add the
tsd_booted_get() function so that tsdn_fetch() can probe whether tsd
bootstrapping is complete and return NULL if not.  All dangerous
conversions of nullable pointers are tsdn_tsd() calls that assert-fail
on invalid conversion.

test/unit/prof_reset.c

@@ -94,7 +94,7 @@ TEST_END
 bool prof_dump_header_intercepted = false;
 prof_cnt_t cnt_all_copy = {0, 0, 0, 0};
 static bool
-prof_dump_header_intercept(tsd_t *tsd, bool propagate_err,
+prof_dump_header_intercept(tsdn_t *tsdn, bool propagate_err,
     const prof_cnt_t *cnt_all)
 {