global: replace verbose strncmp()s with simpler str_begin()s

src/lib-sieve-tool/sieve-tool.c

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

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

src/lib-sieve/plugins/vacation/cmd-vacation.c

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

src/lib-sieve/storage/file/sieve-file-storage.c

src/lib-sieve/storage/ldap/sieve-ldap-db.c

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