Add login process for handling connection requests.

Frees up the host process for handling server-side commands. Login
process sends SIGUSR1 to the host process when activity on the host
socket is detected. The handler then creates the new peer process and
sends SIGUSR1 back to the login process to continue waiting for new
activity.
login.c: Remove continuing message, add comment
Add chat message header in services IPC

Various metadata like index of peer sending the message and the length
of the data segment are important when creating the actual message that
is sent off to peers' clients.
chat.c: Comments of private methods