Having it into another module will prevent the code from being messy
later.

The parsing procedure is taken verbatim: no semantic change, no
behavioural change.

The configuration file defines the default behaviour of pam_u2f.
Individual module invocations under /etc/pam.d can override
settings.

The file-system location of the config file is by default
$sysconfdir/security/pam_u2f.conf, where $sysconfdir is supplied at
build time.

A new module configuration, "conf=", allows to override it at
runtime. Only absolute paths are accepted.

sysconfig -> sconf-dir

- split-input format: add trailing blob for config file

  The corpus needs some update.

- __wrap_open extended to handle the configuration file.

  The configuration file, mutated by the fuzzer, is made
  available to the cfg.c implementation, analogously to
  the handling of authfile_fd.

  The conf_file_path variable will hold the expected file
  name.  If the conf_file= option does not appear in the
  fuzzed argv, the conf_file_path variable is set with
  the CFG_DEFAULT_PATH.

  conf_file_path might also be NULL if the test does not
  involve the use of a configuration file: this happens in
  the fuzz_format_parsers case.

Generate pam_u2f.8.txt from pam_u2f.8.txt.in, replacing SCONFDIR