Added support for optional operators to the byte code implementation.

sieve/tests/comparator.sieve

src/lib-sieve/ext-envelope.c

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

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-code.c

src/lib-sieve/sieve-code.h

src/lib-sieve/sieve-comparators.c

src/lib-sieve/sieve-comparators.h

src/lib-sieve/sieve-generator.c

src/lib-sieve/sieve-validator.c

src/lib-sieve/sieve-validator.h

src/lib-sieve/tst-address.c

src/lib-sieve/tst-header.c

src/lib-sieve/tst-size.c