Fix signature password checking result on abnormal conditions (RhBug:496754)

Fix signature password checking result on abnormal conditions (RhBug:496754)
- Execve() failure wasn't returning an error code, causing rpm to
  think the password was ok when we couldn't even try verifying
- Stricter return code checking from the password checking child:
  the password can only be ok if the child exits with WIFEXITED() *and*
  WIFEXITCODE() of 0. Also WIFEXITCODE() should only be called if
  WIFEXITED() returns true.