Make the child process that failed to execvp exit

Previously the zombie child would continue on the same code path as the forking
parent. Hence one would see spurious "Remove failed" message as the zombie
child would try to perform the same file removal as the parent.

Author: tautschnig

regression/goto-gcc/run_diagnostic/test.desc

@@ -5,5 +5,6 @@ main.i
 ^SIGNAL=0$
 ^execvp /no/such/tool failed: No such file or directory$
 --
+^Remove failed
 ^warning: ignoring
 ^CONVERSION ERROR$

src/util/run.cpp

@@ -133,7 +133,7 @@ int run(
 
       /* usually no return */
       perror(std::string("execvp "+what+" failed").c_str());
-      return 1;
+      exit(1);
     }
     else /* fork() returns new pid to the parent process */
     {