Remove trailing whitespace

Additionally, added a GitHub Action to ensure no more trailing
whitespace will creep in again in the future.

I'm excluding Markdown files from this check, since trailing whitespace
is significant there, and also excluding `build-aux/install-sh` because
there is significant trailing whitespace on the line that sets
`defaultIFS`.

build-aux/install-sh

@@ -115,7 +115,7 @@ fi
 if [ x"$dir_arg" != x ]; then
 	dst=$src
 	src=""
-	
+
 	if [ -d $dst ]; then
 		instcmd=:
 	else
@@ -124,7 +124,7 @@ if [ x"$dir_arg" != x ]; then
 else
 
 # Waiting for this to be detected by the "$instcmd $src $dsttmp" command
-# might cause directories to be created, which would be especially bad 
+# might cause directories to be created, which would be especially bad
 # if $src (and thus $dsttmp) contains '*'.
 
 	if [ -f $src -o -d $src ]
@@ -134,7 +134,7 @@ else
 		echo "install:  $src does not exist"
 		exit 1
 	fi
-	
+
 	if [ x"$dst" = x ]
 	then
 		echo "install:	no destination specified"
@@ -201,17 +201,17 @@ else
 
 # If we're going to rename the final executable, determine the name now.
 
-	if [ x"$transformarg" = x ] 
+	if [ x"$transformarg" = x ]
 	then
 		dstfile=`basename $dst`
 	else
-		dstfile=`basename $dst $transformbasename | 
+		dstfile=`basename $dst $transformbasename |
 			sed $transformarg`$transformbasename
 	fi
 
 # don't allow the sed command to completely eliminate the filename
 
-	if [ x"$dstfile" = x ] 
+	if [ x"$dstfile" = x ]
 	then
 		dstfile=`basename $dst`
 	else
@@ -242,7 +242,7 @@ else
 # Now rename the file to the real destination.
 
 	$doit $rmcmd -f $dstdir/$dstfile &&
-	$doit $mvcmd $dsttmp $dstdir/$dstfile 
+	$doit $mvcmd $dsttmp $dstdir/$dstfile
 
 fi &&