Munge auth: credential filename uniqueness, improved error logging

* Use mkstemp() to create the temporary credential file instead of
  using a filename based upon timestamps.  This solves the problem of 
  munge auth failures due to the credential filenames not being unique.

* Make daughter routines of req_altauthenuser() return PBSE_* codes.
  Replace req_reject() calls in these routines with log_err() calls
  using the aforementioned PBSE codes.  Add log_err() calls in these
  routines for additional error conditions for complete coverage, so
  we can actually tell from the server log what exactly went wrong if
  a munge auth attempt failed.  Call req_reject() once in
  req_altauthenuser() instead.

* Use non-blocking I/O for reading the munge credential in
  pipe_and_read_unmunge().

* Don't delete the credential file if there was a problem unmunge-ing
  the request.