TODO
o Improve Tests (for errors, upper-case/lower-case, for header-fields,
  check, validate all init-parameters)
o Use Tcl-Stub-mechanism
o Use Tcl_Obj-Commands in tclgdbm.c
o Performance, performance, performance (should be improved for qgdbm)
o Use Arrays for representing a gdbm-handle, too. As is done in BLT.