Use $SHELL instead of hardcoding /bin/sh in ratpoison.

Patch by Mark H Weaver <mhw@netris.org>.

--- ratpoison/src/actions.c.orig	2014-09-14 16:46:08.000000000 +0200
+++ ratpoison/src/actions.c	2015-08-06 15:00:54.951575260 +0200
@@ -228,12 +228,12 @@
   add_command ("escape",        cmd_escape,     1, 1, 1,
                "Key: ", arg_KEY);
   add_command ("exec",          cmd_exec,       1, 1, 1,
-               "/bin/sh -c ", arg_SHELLCMD);
+               "$SHELL -c ", arg_SHELLCMD);
   add_command ("execa",		cmd_execa,	1, 1, 1, 
-	       "/bin/sh -c ", arg_SHELLCMD);
+	       "$SHELL -c ", arg_SHELLCMD);
   add_command ("execf",		cmd_execf,	2, 2, 2, 
 	       "frame to execute in:", arg_FRAME,
-	       "/bin/sh -c ", arg_SHELLCMD);
+	       "$SHELL -c ", arg_SHELLCMD);
   add_command ("fdump",         cmd_fdump,      1, 0, 0,
                "", arg_NUMBER);
   add_command ("focus",         cmd_next_frame, 0, 0, 0);
@@ -367,7 +367,7 @@
   add_command ("unsetenv",      cmd_unsetenv,   1, 1, 1,
                "Variable: ", arg_STRING);
   add_command ("verbexec",      cmd_verbexec,   1, 1, 1,
-               "/bin/sh -c ", arg_SHELLCMD);
+               "$SHELL -c ", arg_SHELLCMD);
   add_command ("version",       cmd_version,    0, 0, 0);
   add_command ("vsplit",        cmd_v_split,    1, 0, 0,
                "Split: ", arg_STRING);
@@ -2671,6 +2671,9 @@
   pid = fork();
   if (pid == 0)
     {
+      char *shell_path;
+      char *shell_name;
+
       /* Some process setup to make sure the spawned process runs
          in its own session. */
       putenv(current_screen()->display_string);
@@ -2700,7 +2703,18 @@
       /* raw means don't run it through sh.  */
       if (raw)
         execl (cmd, cmd, (char *)NULL);
-      execl("/bin/sh", "sh", "-c", cmd, (char *)NULL);
+
+      shell_path = getenv ("SHELL");
+      if (shell_path == NULL)
+        shell_path = "/bin/sh";
+
+      shell_name = strrchr (shell_path, '/');
+      if (shell_name == NULL)
+        shell_name = shell_path;
+      else
+        shell_name++;
+
+      execl(shell_path, shell_name, "-c", cmd, (char *)NULL);
       _exit(EXIT_FAILURE);
     }
 
--- ratpoison/src/events.c.orig	2014-09-14 16:46:23.000000000 +0200
+++ ratpoison/src/events.c	2015-08-06 15:02:17.287578780 +0200
@@ -919,7 +919,7 @@
             {
               /* Report any child that didn't return 0. */
               if (cur->status != 0)
-                marked_message_printf (0,0, "/bin/sh -c \"%s\" finished (%d)",
+                marked_message_printf (0,0, "$SHELL -c \"%s\" finished (%d)",
                                        cur->cmd, cur->status);
               list_del  (&cur->node);
               free (cur->cmd);
--- ratpoison/src/messages.h.orig	2014-04-09 01:22:08.000000000 +0200
+++ ratpoison/src/messages.h	2015-08-06 15:04:14.759583801 +0200
@@ -39,7 +39,7 @@
 
 #define MESSAGE_PROMPT_SWITCH_TO_WINDOW "Switch to window: "
 #define MESSAGE_PROMPT_NEW_WINDOW_NAME  "Set window's title to: "
-#define MESSAGE_PROMPT_SHELL_COMMAND    "/bin/sh -c "
+#define MESSAGE_PROMPT_SHELL_COMMAND    "$SHELL -c "
 #define MESSAGE_PROMPT_COMMAND          ":"
 #define MESSAGE_PROMPT_SWITCH_WM        "Switch to wm: "
 #define MESSAGE_PROMPT_XTERM_COMMAND    MESSAGE_PROMPT_SHELL_COMMAND TERM_PROG " -e "