lib-sieve: storage: ldap: sieve-ldap-db - Move all from_str() functions to sieve-ldap-storage-settings.c

Also make them static. These are only needed there.

testsuite: testsuite-mailstore - Use code_override_fields rather than userdb_fields to initialize mail user

It doesn't belong there.

lib-sieve: storage: file: sieve-file-storage - Use mkdir_get_executable_mode() instead of local static version

Roles were reversed in original code.

This makes the extension of the registered item available at evaluation time,
which is e.g. needed to access the extension context.