Unrevert most of regexp reentrancy abort patch

Unrevert most of regexp reentrancy abort patch

The problem was in:
  * src/syntax.c (update_syntax_table_forward): Propertize even when truncated
which is hence not unreverted.
The rest is:
* src/syntax.h (UPDATE_SYNTAX_TABLE_FORWARD_FAST):
(UPDATE_SYNTAX_TABLE_FAST): Re-introduce.
All callers in regex.c changed back to the _FAST versions.

* test/automated/message-mode-tests.el: Tweak the test to rely on auto
propertization in backward-sexp.