dbg 3.0 release

Release dbg 3.0 2023-08-05

This release contains improvements imported from the mkiocccentry repo toolkit.

Added DBG_INVALID (-1) as an invalid debug level. This value is returned by parse_verbosity() on error.

Improve .gitignore to make it more generic and sort entries.

Updated the dbg(3) man page as per the new parse_verbosity() interface. Added a missing not_reached() to the EXAMPLE.

Rename dbg.a to libdbg.a. To allow for linking in this library from say /usr/local/lib the library file has to be renamed to /usr/local/libdbg.a.

Prevent files with /usr/include and with /usr/local/include from showing up in the dependency list (via make depend).

Improved make install and make uninstall. Add to the Makefile the ${PREFIX} makefile variable so one can change the install location from the default /usr/local.

General improvements to the Makefile.

Added some files under .github to improve the dbg GitHub repo.

Improved the dbg example and test code.

Improved the dbg man pages.