Print an error if any targets do not exist.

This commit fixes the last known bug, that apply-default-acl can be
called on a file that does not exist and no error is output.

A new function, path_accessible(), was added and it uses the
faccessat() POSIX call to check whether or not the current effective
user/group can access a path. We then call the new path_accessible()
on every target given on the command line. If any do not exist, an
error is printed:

  $ ./apply-default-acl derp
  ./apply-default-acl: derp: no such file or directory

Along with this change comes a version bump to v0.0.5 in configure.ac.
Some minor reorganization was done in configure.ac as well.