Fix schlib/check_lib

Created by: jkriege2

This summarizes three smaller bugfixes/improvements. the first one is a major bug, the rest is only minor stuff:

• schlib/checklib.py did not save the library-file after a --fix-run in some cases (because a global counter was reset to 0)
• improved error description in S4.4
• S6.2: improved fixes to KiCAD fields (visibility, copy datasheet link from field to DCM ...)