Closes GH-5548.

This is a backport of fcdc0a6db0ae63fbed9e3828137b899b844623ce
to the PHP-7.3 branch. We need to make sure that OnUpdateString
is also called for a NULL value, otherwise the reset of the encoding
at the end of the request will not work.

I believe I already tried to land this before once, but it didn't
actually end up on the PHP-7.3 branch due to a push conflict that
I only noticed just now.

* PHP-7.2:
  Fix #78656: Parse errors classified as highest log-level

There are a few parts here:
 * opcache should not be blocking signals while invoking compile_file,
   otherwise signals may remain blocked on a compile error. While at
   it, also protect SHM memory during compile_file.
 * We should deactivate Zend signals at the end of the request, to make
   sure that we gracefully recover from a missing unblock and signals
   don't remain blocked forever.
 * We don't use a critical section in deactivation, because it should
   not be necessary. Additionally we want to clean up the signal queue,
   if it is non-empty.
 * Enable SIGG(check) in debug builds so we notice issues in the future.

This passes through syslog message unchanged, without splitting
messages at newlines.

* PHP-7.2:
  Fix bug 76596: phpdbg supports display_errors=stderr