Add option to no mark POP3-retrieved mail as READ

When the option is not set, messages "READ" flag is set when the message is
retrieved via POP3. Setting this option defeats that behavior - as requested
by Keyop via IRC.

Since I'm out of option flags, remove the "receive by user number" option.
The receive by user number option was never a good idea and hopefully no
one is actually depending on/using that setting. Uesr numbers in the alias.cfg
file still work.