Implemented detection of duplicate optional arguments.

sieve/errors/address-part-errors.sieve

sieve/errors/interesting.sieve

sieve/errors/match-type-errors.sieve

src/lib-sieve/sieve-address-parts.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-match-types.c

src/lib-sieve/sieve-validator.c