ed: avoid literal newline in s/// (briandfoy#917)

* Yesterday when editing a file I discovered the escape sequence "\n" was expanded to a newline when used in a substitution command
* This has the unintended result of updating one or more elements of the lines-list to contain multiple newline characters
* GNU and OpenBSD versions treat "\n" as a regular "n"; follow this for compatibility, and to avoid the problem of line x containing many lines

Author: mknos

bin/ed

@@ -550,7 +550,7 @@ sub edSubstitute {
         $PrintLastLine = 1;
     }
 
-    # do the search and substitution
+    $args[0] =~ s/\\n/n/g; # newline is not permitted
     for my $lineno (@adrs) {
         my $evalstring = "\$lines[\$lineno] =~ s$args[0]";