lib-sieve: sieve-storage - Use int for sieve_storage_create() status result

src/lib-sieve/plugins/include/ext-include-common.c

src/lib-sieve/sieve-script.c

src/lib-sieve/sieve-storage.c

src/lib-sieve/sieve-storage.h

src/plugins/imap-filter-sieve/imap-filter-sieve.c