Journal fsize

Fixed journal file size / limit arithmetics when the file size was larger than the imposed limits.
Added unit tests for overfill / fillup and file size checks.

See merge request !311

Removed CRC check from journal

See commit fed2e5c2.

See merge request !310

knotc improvements

Improved `knotc` commands + unified and improved help, man and documentation.

- `reload` and `zonestatus` now take arguments and can be called for particular zones
- fixed logging of incoming command (was printing only the last argument)
- fixed use of RCU in remote command processing.
- updated help (`knotc -h`), man page and doxygen documentation
- warning when no arguments are supplied to `refresh -f` and `signzone` (they require at least one argument)
- warning when arguments are passed to `stop` and `checkconf` (they take no arguments)

Fixes #308

See merge request !309

CRC in the journal is replaced by a placeholder to maintain the
journal format.
Removed the zlib dependency.

- libknot-int exports all symbols
- pkg-config is updated to make the change transparent

- Info about command not taking arguments printed to stdout directly.
- Improved error message for missing argument for command.

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

- Allowed the command zonestatus to take zone names as arguments.
- Optimised code of flush.
- Added missing RCU locks and reorganised a little bit.
- Updated documentation and help.

As of now, the version of the library is consistent with the release.

* improve-signal-handling:
  knotd: handle control interrupts atomically
  review: bool signal variables
  fdset: require pselect(), remove unsafe compat implementation
  knotd: reset umask() when starting
  knotd: properly set signal blocking for new threads
  knotd: fix previously broken signal blocking
  knotd: setup signal at once place
  knotd: remove forced stop from signal handler

Merge request !308