(bug 38110) provide a way to separate out schema changes

(bug 38110) provide a way to separate out schema changes

Allows update.php to be run when $wgAllowSchemaUpdates = false.

This is useful for non-WMF environments where strict DB permissions
allow database updates (which update.php performs), but no schema
changes (such as adding or dropping tables or indices which update.php
also performs).

It does this by adding the --schema and --noschema flags. Without
either of these flags, update.php will perform exactly as before.

With --noschema, all changes to the table structure or table additions
are skipped. Only data changes are made.

With --schema is used, no schema changes are made to the database, but
the schema changes are saved to a separate SQL file that can be run.

Change-Id: I96b4cfd4c02e9cbf46cc6a0499b87fb3b89020a0