Envelope: working towards proper RFC compliance of forward/return-path parsing.

TODO

doc/rfc/RFC Controversy.txt

src/lib-sieve/ext-envelope.c

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

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

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

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

src/lib-sieve/sieve-address.c

src/lib-sieve/sieve-address.h

src/lib-sieve/sieve-common.h

tests/extensions/envelope.svtest

tests/testsuite.svtest