makefile: clean duplicates
makefile: merge libknotcs -> libknot
makefile: generate pkg-config file for libknot, versioning

As of now, the version of the library is consistent with the release.
pkg-config: don't publish already prefixed include path
Makefile: clean duplicate headers [review]
Makefile: fix broken linking of systemd [review]
pkg-config: use PKG_INSTALLDIR [review]
Merge branch 'install_headers' into 'master'

Enables libknot headers and .pc installation, allowing use of libknot in
external projects.

* install_headers:
  pkg-config: use PKG_INSTALLDIR [review]
  Makefile: fix broken linking of systemd [review]
  Makefile: clean duplicate headers [review]
  pkg-config: don't publish already prefixed include path
  makefile: generate pkg-config file for libknot, versioning
  makefile: merge libknotcs -> libknot
  makefile: clean duplicates