Compiler now warns about syntactically invalid header field names.

TODO

src/lib-sieve/rfc2822.c

src/lib-sieve/rfc2822.h

src/lib-sieve/sieve-commands.c

src/lib-sieve/sieve-commands.h

src/lib-sieve/tst-address.c

src/lib-sieve/tst-exists.c

src/lib-sieve/tst-header.c

tests/compile/warnings/invalid-headers.sieve