config: add $egrep

Introduce a new variable set in $PLAN9/config:
$egrep, which is conditionally set to either
`egrep` or `grep -E`, based on platform, and use
this when we want to invoke `egrep`.

This works around the endless warnings from GNU
grep stating that `egrep` is deprecated in favor
of `grep -E`, when run on e.g. Linux.

dist/checkman.awk

@@ -446,7 +446,7 @@ END {
 			gsub("\\(", " \\(", b)
 			gsub("\\)", "\\)", b)
 			split(tolower(i), a, "/")
-			print "egrep -in '^\\.IR.*" b "' $PLAN9/man/man*/* # Need " tolower(i) |"sort"
+			print "grep -in '^\\.IR.*" b "' $PLAN9/man/man*/* # Need " tolower(i) |"sort"
 		}
 	}
 	close("sort")