lib-sieve: sieve - Use int for sieve_open_script() status result

src/lib-sieve/sieve.c

src/lib-sieve/sieve.h

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

src/plugins/imapsieve/imap-sieve.c

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