lib-sieve: sieve-extensions - Use int for sieve_extension_require() status result

src/lib-sieve/plugins/environment/sieve-ext-environment.h

src/lib-sieve/plugins/imap4flags/sieve-ext-imap4flags.h

src/lib-sieve/plugins/vacation/ext-vacation-seconds.c

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/plugins/imapsieve/ext-imapsieve.c