Functions to start/top the rdm process from emacs.

This functions uses the function `rtags-includes-func` for settings the includes to the rdm process. The default function just return the variable `rtags-rdm-includes`. The following function are defined:

rtags-start-process-maybe : start the rdm process if not started.
rtags-cancel-process : stop the rdm process.
rtags-restart-process : stop and start the rdp process.