These are needed for autoreconf

Also rename associated variables and functions.

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.