- When doing the selinux change, I noticed that there was a lot of
  duplication of code in handing string configuration items, so I
  refactored this into a common function.

- Added a config_string_max that can be passed a maximum acceptable
  length, used to limit ttydir to NAME_MAX.

- The behavior of config_seccomp was different than other strings: if the
  item was already defined, then the second attempt to set it would fail
  instead of just replacing the value. Changed to just replace the value.

- Remove unused key and lxc_conf arguments to config_path_item().
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>