Improve runguard error message when command cannot be executed

When the system error is "Permission denied" this typically means
that the full path of the command is not traversible for the user.
This can happen for example when a validator is executed as
`domjudge-run` user and `/home/domjudge` does not have `o+x` set.

(cherry picked from commit aa67a2b)

Author: eldering

judge/runguard.cc

@@ -815,6 +815,20 @@ int groupid(char *name)
 	return (int) grp->gr_gid;
 }
 
+char *username()
+{
+	int saved_errno = errno;
+	errno = 0; /* per the linux GETPWNAM(3) man-page */
+
+	struct passwd *pwd;
+	pwd = getpwuid(getuid());
+
+	if ( pwd==nullptr || errno ) error(errno,"failed to get username");
+	errno = saved_errno;
+
+	return pwd->pw_name;
+}
+
 long read_optarg_int(const char *desc, long minval, long maxval)
 {
 	char *ptr;
@@ -1426,9 +1440,7 @@ int main(int argc, char **argv)
 
 		/* And execute child command. */
 		execvp(cmdname,cmdargs);
-		struct rlimit limit;
-		getrlimit(RLIMIT_NPROC, &limit);
-		error(errno,"cannot start `%s', limit: %ld/%ld | ",cmdname, limit.rlim_cur, limit.rlim_max);
+		error(errno,"cannot start `%s' as user `%s'", cmdname, username());
 
 	default: /* become watchdog */
 		verbose("child pid = %d", child_pid);