patch #766650 - whichdb not identifying dbm DBs when dbm linked with gdbm

patch #766650 - whichdb not identifying dbm DBs when dbm linked with gdbm

At this point, the problem appears particular to the OS/2 EMX port of
gdbm (which is at v1.7.3) - this combination produces a .pag file but
no .dir file.

A more sophisticated patch which checks magic numbers when dbm.library
indicates that dbm is linked to gdbm, and there is no .dir file, is
still attached to the above patch entry for reconsideration after 2.3
is released.

This checkin applies a workaround specific to the known failure case.