Add empty lines to ipc.h

For better readability.
Enable remote commands with ENABLE_REMOTE_CMDS

Preparation for source code.
Update main.c:quit_on_invalid_arg() function

 * Add comment.
 * Rename to quit_on_arg_parsing().
 * Include <stdlib.h> for exit().
Add --enable-remote-cmd configure switch
Document --enable-remote-cmd configure switch
Merge branch 'allow-disabling-remote-cmds'

Add --enable-remote-cmd switch to configure script disabled by default,
as current implementation of the feature is insecure.