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.

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.

pop3c: Added pop3c_quick_received_date setting. With this enabled dsync no longer requires calling TOP for each message to get the metadata.

Moved ssl_client_ca_dir and ssl_crypto_device to mail_storage_settings. Probably could also be moved to master_service_settings, but that makes accessing them a bit more tricky with the current code.

lib-storage: Replaced pop3c_ssl_ca_dir and imapc_ssl_ca_dir with generic ssl_client_ca_dir.

Reverted 269104a0821b (Build imapc and pop3c always as plugins.) This is no longer needed now that lib-ssl-iostream can be linked without linking openssl.

Build imapc and pop3c always as plugins. This avoids having to link openssl library to all binaries that use lib-storage. It appears that simply by linking with openssl causes each such process to use 100-200 kB of more memory. With 10k imap processes this is 1-2 GB of wasted memory. The imapc and pop3c are still registered as stubs, and their plugins are automatically loaded when needed.

pop3c: Added pop3c_master_user setting.

lib-storage: Added initial support for pop3c backend.