qrexec: start process in a login shell

Prepend "-" to shell name, to instruct it being a login shell. This way
shell will initialize environment, load /etc/profile etc.

Fixes QubesOS/qubes-issues#2903

qrexec/qrexec-agent.c

@@ -211,7 +211,7 @@ void do_exec(const char *cmd)
             setsid();
             /* This is a copy but don't care to free as we exec later anyways.  */
             env = pam_getenvlist (pamh);
-            execle("/bin/sh", "sh", "-c", realcmd, (char*)NULL, env);
+            execle("/bin/sh", "-sh", "-c", realcmd, (char*)NULL, env);
             exit(127);
         default:
             /* parent */