* Remove the automatic conversion of NULL and '\0' to 0.  Add a flag
  for the cases that require lax evaluation.

* Make trailing characters generate an error by default for the
  mutt_atouX functions.  Add a flag for that case.  Most of the IMAP
  code parses numbers out of a stream, so add the flag to those
  calls.

* The mutt_atouX functions were also behaving incorrectly with
  invalid input, e.g. "ABC", returning and setting 0.  Fix them to
  return an error in those cases.

* Add a mutt_atoll() function, to be used in the next commit.

* Change converters to store 0 on error.  atos, atoi, and atoui were
  already doing this, but the others were not.