implement TOP command from POP3

[jduepmeier]

the TOP command is an optional command to receive only a limited number of lines from a mail body (https://tools.ietf.org/html/rfc1939#page-11).

I think this will help to speed up mail fetching where the body is not important only header lines.

[cbdevnet]

Seems doable, I'll look into it!

Will require addition of the command to https://github.com/cmail-mta/cmail/blob/master/cmail-popd/popstatemachine.c (state_transaction) as well as addition of a handler to https://github.com/cmail-mta/cmail/blob/master/cmail-popd/popfunctions.c.

Due to the escape character required in the protocol, it might be useful to extend pop_retr with an additional parameter for the number of lines instead of implementing the bytestuffing again. This would require also introducing some rudimentary parsing to separate at least the header section from the data section (as the RFC specifies that the argument is taken to mean "headers + n lines").