lib-sieve: sieve-extensions - Notify caller of sieve_extensions_set_string() about extension load failures

Since the script storage code is widely engrained in the Pigeonhole sources,
this is a very large commit that cannot be reduced further into smaller
preparatory bits.

Produced binary corruption error before.

lib-sieve: storage: file: sieve-file-storage - Fully stat both storage and active path in sieve_file_storage_autodetect()

Makes the autodetection more reliable and avoids later failures.

sieve_storage_do_create_personal() will go away.

Should make later commit yield clearer diff.

Later, this is needed to allow retrieving a global script from one of many
storages, rather than just one.

lib-sieve: storage: ldap: sieve-ldap-storage-settings - Use settings as parameter to sieve_ldap_settings_check() directly

lib-sieve: storage: file: sieve-file-storage - Move debug logging in sieve_file_storage_autodetect()

lib-sieve: storage: file: sieve-file-storage - Move storage creation in sieve_file_storage_autodetect()

lib-sieve: storage: file: sieve-file-storage - Perform write check earlier in sieve_file_storage_do_autodetect()

lib-sieve: storage: file: sieve-file-storage - Merge sieve_file_storage_autodetect() into sieve_file_storage_do_init_default()

For now, these yield the same result.