build: mkreq: rm duplicate -lfoo flags (re: d01798f, 7d75ed4)

(See src/cmd/INIT/README-mamake.md for info on *.req files.)

I've been noticing a lot of duplicate -l flags to the compiler in
the build log, e.g. on macOS, '-last' and '-liconv' repeated three
times when linking ksh. This inelegance is due to how the *.req
files are generated. Now that the code to do that has been
consolidated into a couple of scripts, we can tweak this quite
easily and sanitise the *.req files (which are installed and used
as arch/*/lib/lib/* files).

For example, src/cmd/ksh93/shell.req a.k.a. arch/*/lib/lib/shell,
contents before this change (on macOS):

 -lshell
 -ldll -last -liconv
 -lcmd -lutil -last -liconv
 -last -liconv
 -lnetwork

...and after:

 -lshell
 -last
 -lcmd
 -ldll
 -liconv
 -lnetwork
 -lutil

src/cmd/INIT/mkreq-maplib.sh:
- At the end, do not insert an extra space before the '-l' output;
  a leading space is already present.

src/cmd/INIT/mkreq.sh:
- Instead of 'case' and 'echo', use 'grep' to look for lines that
  start with a space and '-l' (the dependencies of the current
  library). This keeps the results on one line each.
- Pipe the resulting lines through 'sort -u' to remove duplicates.

src/cmd/INIT/README-mamake.md

@@ -203,8 +203,8 @@ The effect of `bind` is global, not scoped; it takes effect for all commands
 physically following it, regardless of `make`…`done` nesting level.
 
 There is also a mechanism to communicate library dependency information across Mamfiles and `mamake` invocations.
-If a file named *libraryname* in the current directory
-or an `${INSTALLROOT}/lib/lib/`*libraryname*`.req` file
+If a file named *libraryname*`.req` in the current directory
+or an `${INSTALLROOT}/lib/lib/`*libraryname* file
 exists, `mamake` processes each of the words in the form `-l`*libraryname* in its contents
 as if they were arguments to `bind` commands
 and the resulting values are appended to the value of `mam_lib`*libraryname*

src/cmd/INIT/mkreq-maplib.sh

@@ -68,7 +68,7 @@ trap 'set +o noglob; rm -rf "$req".exe*' 0
 ######### Main ##########
 #  Generate .req file.  #
 #########################
-r='-'
+r=' -'
 for i in $src
 do	if	$allcc -c $i >/dev/null 2>&1
 	then	rm $(basename $i .c).o &
@@ -89,4 +89,4 @@ do	if	$allcc -c $i >/dev/null 2>&1
 		done
 	fi
 done
-echo " $r" > $req.req
+echo "$r" > $req.req

src/cmd/INIT/mkreq.sh

@@ -2,7 +2,7 @@
 #                                                                      #
 #              This file is part of the ksh 93u+m package              #
 #          Copyright (c) 1984-2012 AT&T Intellectual Property          #
-#          Copyright (c) 2020-2023 Contributors to ksh 93u+m           #
+#          Copyright (c) 2020-2024 Contributors to ksh 93u+m           #
 #                      and is licensed under the                       #
 #                 Eclipse Public License, Version 2.0                  #
 #                                                                      #
@@ -95,13 +95,10 @@ exec >$self.req
 echo " -l$self"
 for name
 do	if	test -f $INSTALLROOT/lib/lib/$name
-	then	y=$(cat $INSTALLROOT/lib/lib/$name)
-		case $y in
-		*-?*)	echo "" $y ;;
-		esac
+	then	grep '^ -l.' $INSTALLROOT/lib/lib/$name
 		continue
 	elif	test ! -f $INSTALLROOT/lib/lib$name.a
 	then	try_to_link $name -L$INSTALLROOT/lib || try_to_link $name || continue
 	fi
 	echo " -l$name"
-done
+done | sort -u