Fix the NC_INMEMORY code to work in all cases with HDF5 1.10.

[DennisHeimbigner]

re: github issue #1111

One of the less common use cases for the in-memory feature is
apparently failing with HDF5-1.10.x. The fix is complicated and
requires significant changes to libhdf5/nc4memcb.c. The current
setup is detailed in the file docs/inmeminternal.dox.

Additionally, it was discovered that the program
nc_test/tst_inmemory.c, which is invoked by
nc_test/run_inmemory.sh, actually was failing because of the
above problem. But the failure is not detected since the script
does not return non-zero value.

Other Changes:

1. Fix nc_test_tst_inmemory to return errors correctly.
2. Make ncdap_tests/findtestserver.c and dap4_tests/findtestserver4.c
   be generated from ncdap_test/findtestserver.c.in.
3. Make LOG() print output to stderr instead of stdout to
   avoid contaminating e.g. ncdump output.
4. Modify the handling of NC_INMEMORY and NC_DISKLESS flags
   to properly handle that NC_DISKLESS => NC_INMEMORY. This
   affects a number of code pieces, especially memio.c.