The userland constants do not start with PHP_

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Promote warnings to exceptions in ext/gettext, ext/sysvmsg and ext/xml

Closes GH-5926

Tests for the default case of the parameter of xml_parser_get_option() and xml_parser_set_option(). These cases are, as of now, not being tested.