BUG/MEDIUM: mworker: Fix re-exec when haproxy is started from PATH

If haproxy is started using the name of the binary only (i.e.
not using a relative or absolute path) the `execv` in
`mworker_reload` fails with `ENOENT`, because it does not
examine the `PATH`:

  [WARNING] 315/161139 (7) : Reexecuting Master process
  [WARNING] 315/161139 (7) : Cannot allocate memory
  [WARNING] 315/161139 (7) : Failed to reexecute the master processs [7]

The error messages are misleading, because the return value of
`execv` is not checked. This should be fixed in a separate commit.

Once this happened the master process ignores any further
signals sent by the administrator.

Replace `execv` with `execvp` to establish the expected
behaviour.

This bug was introduced in commit 73b85e75b3963086be889e1fb40a59e7ef2ad63b.
diff --git a/src/haproxy.c b/src/haproxy.c
index a22ed32..b4f6a4f 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -650,7 +650,7 @@
 
 	deinit(); /* we don't want to leak FD there */
 	Warning("Reexecuting Master process\n");
-	execv(next_argv[0], next_argv);
+	execvp(next_argv[0], next_argv);
 
 alloc_error:
 	Warning("Cannot allocate memory\n");