ps: new format option 'exe' now shows executable path if unavail from cmdline

Red Hat Bugzilla #1399206

ps/ps.1

@@ -1251,6 +1251,14 @@ option can be used to force the decimal representation.  (alias
 .BR uname ", "  user ).
 T}
 
+exe	EXE	T{
+path to the executable. Useful if path cannot be printed via
+.BR cmd ", " comm
+or
+.BR args
+format options.
+T}
+
 f	F	T{
 flags associated with the process, see the
 .B PROCESS FLAGS