build-aux/manconv.sh: Fix US-ASCII and UTF-8 output.

groff defaults to SGR escapes. Using -P-c passes -c to grotty
which restores the old behavior. Perhaps there is a better way to
get pure plain text output but this works for now.

(cherry picked from commit 6a1093c0004c42eeaef312456c295671496dd67a)

build-aux/manconv.sh

@@ -38,10 +38,10 @@ s/^\\.PD\$/.PD $PD/"
 
 case $FORMAT in
 	ascii)
-		groff -t -mandoc -Tascii | col -bx
+		groff -t -mandoc -Tascii -P-c | col -bx
 		;;
 	utf8)
-		groff -t -mandoc -Tutf8 | col -bx
+		groff -t -mandoc -Tutf8 -P-c | col -bx
 		;;
 	ps)
 		sed "$SED_PD" | groff -dpaper=$PAPER -t -mandoc \