lib-sieve: Consolidated checking mailbox name.

src/lib-sieve/ext-fileinto.c

src/lib-sieve/plugins/mailbox/tst-mailboxexists.c

src/lib-sieve/plugins/metadata/tst-metadata.c

src/lib-sieve/plugins/metadata/tst-metadataexists.c

src/lib-sieve/sieve-actions.c

src/lib-sieve/sieve-actions.h