Check for presence of SHELL environment variable

- POSIX states the SHELL environment variable "... shall represent a
  pathname of the user's preferred command language interpreter." As
  such, st should check for its presence when deciding what shell to
  use; just as HOME can be defined to override one's passwd-defined home
  directory, a user should also be able to override their passwd-defined
  shell using the SHELL environment variable.

st.c

@@ -1158,7 +1158,10 @@ execsh(void) {
 			die("who are you?\n");
 	}
 
+	if (!(sh = getenv("SHELL"))) {
 		sh = (pw->pw_shell[0]) ? pw->pw_shell : shell;
+	}
+
 	if(opt_cmd)
 		prog = opt_cmd[0];
 	else if(utmp)