Include string error when open fails
The binary_format_minor_version can be 0 so we can't really check it
Move debugging macros to maxminddb.c and start refactoring debug output

I'm trying to make it more like the Perl reader/decoder - the debugging output
there is more thorough and was very useful when debugging problems with that
code.
Since we're going to read or mmap the entire file, just use that to find metadata

Previously we were reading the last 128kb of the file into a separate variable
and using that to find the metadata, but that was kind of pointless. We have
to read/mmap the whole file anyway.

I also simplified the metadata reading code a little while I was in there.
Only use MMDB_aget_value internally

All MMDB_Xget_value calls end up there eventually, so this should be a little
quicker.
Ran uncrustify