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.

lib-storage: Added initial support for pop3c backend.