modemmanager: added new proto options

modemmanager: added new proto options

Added signal refresh rate option
modemmanager: update readme.md
Added description for added proto options.

Added compile option to compile --with-at-command-via-dbus for allowing
AT commands to modem without --debug flag

 Changes to be committed:
	modified:   net/modemmanager/Config.in
	modified:   net/modemmanager/Makefile
	modified:   net/modemmanager/files/modemmanager.init
	modified:   net/modemmanager/files/modemmanager.proto
	modified:   README.md

Signed-off-by: Valtteri Holopainen <redacted>