pop3c: Add no pipelining pop3c feature This should help with certain broken pop3c servers that advertise that they support pipelining but they really don't.
pop3c: Added full support for running commands asynchronously (with PIPELINING)
Changed type of internet port values to in_port_t everywhere. Created special SET_IN_PORT setting type for internet port values. Created net_str2port() for parsing internet port values. Removed several atoi() invocations in the process.
Added ssl_client_ca_file to specify the CA certs as a file instead of as a dir. This is required for Redhat-based systems where there isn't a CA directory like in Debian/Ubuntu.
lib-storage: Added initial support for pop3c backend.