add short_open_tag runtime option

Surprisingly, it was missing.
http://php.net/manual/en/ini.core.php#ini.short-open-tag

- allow short echo tag regardless of short_open_tag

According to http://php.net/manual/en/language.basic-syntax.phpmode.php,
short echo tags are always recognized and valid even when we're not
allowing short tags. Don't check for it in the lexer.

- add EnableShortTags to unit md5

Since short_open_tags changes lexer behavior, we have to add it to our
md5 hash. Fixes test failures where short_tags.00[1,2].php were
identical except for their ini settings.