Introduce SUPPORTCAPA and SUPPORTAUTH config commands to allow further
pop3 test server expansion for tests that require CAPA or AUTH support,
although this will need some extra work to make it fully functional.
With commit 035ef06bda applied, the test pop3 server needs to send
".\r\n" as the body terminating sequence and there needs to be a final
CRLF in the actual body in the test data file.
