Variables: First work towards match value support.

src/lib-sieve/ext-envelope.c

src/lib-sieve/plugins/body/tst-body.c

src/lib-sieve/plugins/imapflags/tst-hasflag.c

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

src/lib-sieve/plugins/variables/ext-variables-common.h

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

src/lib-sieve/plugins/variables/tst-string.c

src/lib-sieve/plugins/variables/variables-match.sieve

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

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

src/lib-sieve/tst-address.c

src/lib-sieve/tst-header.c