Added conflict and duplicate checking to vacation and reject actions.

sieve/errors/action-conflicts.sieve

sieve/errors/action-duplicates.sieve

src/lib-sieve/cmd-discard.c

src/lib-sieve/cmd-keep.c

src/lib-sieve/cmd-redirect.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

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

src/lib-sieve/sieve-actions.c

src/lib-sieve/sieve-actions.h

src/lib-sieve/sieve-commands-private.h

src/lib-sieve/sieve-interpreter.c

src/lib-sieve/sieve-interpreter.h

src/lib-sieve/sieve-result.c

src/lib-sieve/sieve-result.h