This patch adds machinary to create new warning flags using the underscore
"_" prefix. This is necessary because all letters of the alphabet and letters
prefixed with a dot "." are used already.

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-09-08  Justin Squirek  <squirek@adacore.com>

	* switch-c.adb (Scan_Front_End_Switches): Add new warning switch
	case to handle underscore flags.
	* warnsw.adb, warnsw.ads (Set_Underscore_Warning_Switch): Create
	new procedure to handle underscores.