squash! [sieve] add parser support for imap4flags

Redefine copy to not allow an empty case to prevent grammar conflicts.
(Required defining rtags for redirect aciton.)
fixup! [sieve] add parser support for imap4flags
[sieve] Anticipate bytecode changes for VARIABLES extension

Combined generation of bytecode for hasflag test and header test into
a single case.

The bytecode changes for the set/add/remove-flag actions will be done
at a later date, since they will be re-definitions anyway.