Cleaned up mail storage-related code (modified patch by Timo Sirainen).

src/lib-sieve/plugins/mailbox/tag-mailbox-create.c

src/lib-sieve/plugins/mailbox/tst-mailboxexists.c

src/lib-sieve/sieve-actions.c

src/lib-sieve/sieve-actions.h

src/lib-sieve/sieve-types.h

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

src/sieve-tools/sieve-filter.c

src/sieve-tools/sieve-test.c

src/testsuite/testsuite-mailstore.c

src/testsuite/testsuite-mailstore.h

src/testsuite/testsuite-script.c

src/testsuite/testsuite.c