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'
debian: fix package build, broken by c5e2007.

Auto generated manpages are in different locations than static ones, so
libsmack-dev.manpages needs to be modified.
It now includes all generated section 3 manuals.
doc: fix parallel build, broken by c5e2007.

Parallel make failed, because dependencies were not properly specified
for all auto generated man pages.
doc: show version

Fix documentation to show correct package version.