parse-number: change return value to gint64 instead of glong

Using glong causes potential portability issues as somewhere it is 64bits,
on other platforms it is only 32 bits.

The lexer, which uses the parse-number functions, stores the return value in
gint64, that is always 64 bits, so use that always.

Reported-By: Tamayo, Andres <Andres.Tamayo@viasat.com>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
test_parse_number: add testcase for a real 64bit value

parse-number had an issue on 32 bit big endian platforms, make sure it is
actually working with 64 bit values, always by adding a testcase that
expects a 64 bit value.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>