Implemented Sieve settings handling (debug not working yet).

src/lib-sieve/Makefile.am

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

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

src/lib-sieve/sieve-settings.c

src/lib-sieve/sieve-settings.h

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

src/testsuite/testsuite-script.c

src/testsuite/testsuite.c