RT#83257: use AnyDBM_File rather than hardcode GDBM_File

To improve portability, use AnyDBM_File to select a DBM
rather than rely on GDBM_File.  GDBM_File apparently used
to be a core module, but not all distributions included it,
particularly OS X.  In any event, GDBM_File is no longer
core.

This patch also includes a tweak to allow MARC::Charset to
work with NDBM_File and ODBM_File, neither of which
support 'exists'.

I've tested MARC::Charset successfully on the following
DBMs:

- GDBM_File
- DB_File
- NDBM_File
- ODBM_File
- SDBM_File

This is also my preferred order; SDBM_File is selected last
because it produces the biggest data file on disk.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
minor tweaks to MARC::Charset Makefile.PL

- add myself to author line
- explicitly set license

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
remove META.yml -- let Makefile.PL deal with creating CPAN metadata

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>