lib-sieve: util: program-client-local: Changed link between parent and child process from a socketpair to a double pipe.

Turns out /dev/stdin, /dev/stdout, and /dev/fd/XX don't work on Linux if the underlying FD is a socket.
Since some programs may rely on those symbolic links to work, the local client is amended to use two pipes instead.
This will not help for programs invoked in the remote script service, since a socket is required for the connection.

Sieve extprograms plugin: Made line endings configurable for the input passed to the external program.

lib-sieve: util: program client: Fixed handing of situation when no input is provided to external program used as a command.
Previous fixes still omitted a few important aspects:
Connect return result wasn't actually passed up to program_client_run().
Default exit code was wrong.

Recent changes in handling of unknown tags caused `make test-plugins' to fail.

lib-sieve: spamtest/virustest extensions: Tests were erroneously performed against the original message.
When used together with extprograms filter to add the spam headers, the changes were not being used by the spamtest and virustest extensions.

Testsuite hostname is now true local hostname.

It is still a plugin, but no longer a separate package.