lib-sieve: added support for substituting the entire message.

src/lib-sieve/cmd-discard.c

src/lib-sieve/cmd-keep.c

src/lib-sieve/cmd-redirect.c

src/lib-sieve/ext-reject.c

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

src/lib-sieve/plugins/enotify/cmd-notify.c

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

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

src/lib-sieve/sieve-actions.c

src/lib-sieve/sieve-message.c

src/lib-sieve/sieve-message.h

src/lib-sieve/sieve-result.c

src/lib-sieve/sieve-result.h

src/testsuite/testsuite-message.c