Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Implement nullsafe ?-> operator

RFC: https://wiki.php.net/rfc/nullsafe_operator

Closes GH-5619.

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>