gsed: add libexec/gnubin option for unprefixed sed

Mirrors what is done for coreutils.

Closes Homebrew/legacy-homebrew#20536.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/gnu-sed.rb

@@ -14,11 +14,25 @@ def install
 
     system "./configure", *args
     system "make install"
+
+    (libexec/'gnubin').install_symlink bin/"gsed" =>"sed"
+    (libexec/'gnuman/man1').install_symlink man1/"gsed.1" => "sed.1"
   end
 
   def caveats; <<-EOS.undent
     The command has been installed with the prefix 'g'.
     If you do not want the prefix, install using the 'default-names' option.
+
+    If you need to use these commands with their normal names, you
+    can add a "gnubin" directory to your PATH from your bashrc like:
+
+        PATH="#{opt_prefix}/libexec/gnubin:$PATH"
+
+    Additionally, you can access their man pages with normal names if you add
+    the "gnuman" directory to your MANPATH from your bashrc as well:
+
+        MANPATH="#{opt_prefix}/libexec/gnuman:$MANPATH"
+
     EOS
   end