This patch implements new functions and improves compatibility with
new versions of GDBM.
* ext/GDBM_File/GDBM_File.xs: Define interface methods for
functions in newer GDBM versions.
(GDBM_version): New static method.  Return the version number
(string in scalar, array of numbers in list context).  Provide
heurisics for determining the library version for GDBM prior
to 1.9.
(gdbm_close): Propagate return value from the library call.
(gdbm_DESTROY): Croak if closing the database fails.
(gdbm_UNTIE): New method.
(gdbm_FETCH): Check database validity. Croak if gdbm_fetch returns
error (except GDBM_ITEM_NOT_FOUND).
(gdbm_STORE,gdbm_DELETE): Likewise.
(gdbm_FIRSTKEY,gdbm_NEXTKEY): Likewise.
(gdbm_EXISTS): Check database validity.
(gdbm_errno): New function.
(gdbm_syserrno): New function.
(gdbm_strerror): New function.
(gdbm_clear_error): New function.
(gdbm_needs_recovery): New function.
(gdbm_recover): New function.
(gdbm_count): New function.
* ext/GDBM_File/typemap (gdbm_count_t): Map to T_COUNT.
* ext/GDBM_File/GDBM_File.pm: Document everything. Raise $VERSION.
* MANIFEST: Update.
* AUTHORS: Update.

* ext/GDBM_File/GDBM_File.xs (rcvr_errfun): Use dTHX
(gdbm_GDBM_version): Don't declare package.
* ext/GDBM_File/typemap: Map gdbm_count_t to T_UV.
* ext/GDBM_File/t/count.t: Use File::Temp to create db in
a temporary directory.
* ext/GDBM_File/t/fatal.t: Likewise.
* ext/GDBM_File/t/opt.t: Likewise.