Dissolve libknots between libknot and knotd

The functions in the static library libknots (src/common/*) can be probably split between libknot and knotd.

Some functions (e.g., debugging) may be left there. But hopefully only stuff which are internally.

Notes:

• libknot must not have any dependency on headers outside src/libknot.
• common/errcode.h is needed in libknot, otherwise the library will be unusable.