Fix tsd cleanup regressions.

Fix tsd cleanup regressions that were introduced in
5460aa6f6676c7f253bfcb75c028dfd38cae8aaf (Convert all tsd variables to
reside in a single tsd structure.).  These regressions were twofold:

1) tsd_tryget() should never (and need never) return NULL.  Rename it to
   tsd_fetch() and simplify all callers.
2) tsd_*_set() must only be called when tsd is in the nominal state,
   because cleanup happens during the nominal-->purgatory transition,
   and re-initialization must not happen while in the purgatory state.
   Add tsd_nominal() and use it as needed.  Note that tsd_*{p,}_get()
   can still be used as long as no re-initialization that would require
   cleanup occurs.  This means that e.g. the thread_allocated counter
   can be updated unconditionally.
Don't force TLS on behalf of heap profiling.

Revert 6716aa83526b3f866d73a033970cc920bc61c13f (Force use of TLS if
heap profiling is enabled.).  No existing tests indicate that this is
necessary, nor does code inspection uncover any potential issues.  Most
likely the original commit covered up a bug related to tsd-internal
allocation that has since been fixed.
Make prof-related inline functions always-inline.