doveadm-sieve: Fixed mtime updates.

src/lib-sievestorage/sieve-storage-private.h

src/lib-sievestorage/sieve-storage-save.c

src/lib-sievestorage/sieve-storage-script.c

src/lib-sievestorage/sieve-storage-script.h

src/lib-sievestorage/sieve-storage.c

src/managesieve/cmd-setactive.c

src/plugins/doveadm-sieve/doveadm-sieve-plugin.c