This was acomplished by making errSymbolAdd add the error symbol directly
into the global hash table and removing errnumlist which is not needed
anymore.
Unit tests were added for checking the following cases:
- Adding a valid symbol and checking that it exists (fixed by this change)
- Getting an existing error symbol
- Getting a non existing error symbol
- Adding an invalid error symbol (fixed by this change)
- Adding an error symbol with a code that already
exists (fixed by this change)
Therefore, issue #268 was fixed
In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".
Makefiles and the build system were deliberately excluded.
