Add PIE and RELRO to binaries

MODULE_LIBS is now part of LIBDOVECOT. No need to explicitly link it everywhere.

Added support for a simplified IPC infrastructure. The idea is that you have one "ipc" proxy process, where all server processes connect to. IPC clients can then connect to the proxy and ask it to forward commands to either a specific server or all servers. The proxy does this, and forwards back any replies from the server.