Fixed copyright clauses.

Checked that copyright notices are properly set up as according to:

http://www.gnu.org/licenses/gpl-howto.html

Corrected where they are not. Note that one or two-line fixes do not
count as copyrightable assets.

Additionally, fixed a minor indentation issue in chsmack.c.
Use strncpy() always copying labels.

Defence in depth and a good practice. Places an absolute limit
to the length copied.
Merge remote-tracking branch 'jarkkos/copyright-fix'
Merge remote-tracking branch 'jarkkos/strcpy-fix'
Helper function get_label()

The helper function ssize_t get_label(char *dest, const char *src)
validates the given label and copies it to the dest buffer if
available.
Copy labels using get_label()

Copy and validate labels in a single transaction:

- No trust for having '\0' in the src buffer when copying
  labels.
- Improves performance by combining length calculation, validation
  and copying.
Generate API documentation by using Doxygen.

Generate API documentation from smack.h by using Doxygen so that
documentation needs to be maintained only in one place.
Solve problem with "make distcheck" breaking on systemd config files.

Dist packages created by "make dist" were built without smack.mount and
smack.service files. This caused the packages to fail to build.
Adding the files to EXTRA_DIST solves the problem.
Merge remote-tracking branch 'jsakkine/issue45'