Fix handling of corrupt files + proper offset handling for hdf5 files

[DennisHeimbigner]

re e-support UBS-599337
re pull request #405
re pull request #446

Notes:

1. This branch is a cleanup of the magic.dmh branch.
2. magic.dmh was originally merged, but caused problems with parallel IO.
   It was re-issued as pull request Better detection of corrupt hdf5, second take.  #446.
3. This branch + pull request replace any previous pull requests and magic.dmh branch.

Given an otherwise valid netCDF file that has a corrupted header,
the netcdf library currently crashes. Instead, it should return
NC_ENOTNC.

Additionally, the NC_check_file_type code does not do the
forward search required by hdf5 files. It currently only looks
at file position 0 instead of 512, 1024, 2048,... Also, it turns
out that the HDF4 magic number is assumed to always be at the
beginning of the file (unlike HDF5).
The change is localized to libdispatch/dfile.c See
https://support.hdfgroup.org/release4/doc/DSpec_html/DS.pdf

Also, it turns out that the code in NC_check_file_type is duplicated
(mostly) in the function libsrc4/nc4file.c#nc_check_for_hdf.

This branch does the following.

1. Make NC_check_file_type return NC_ENOTNC instead of crashing.
2. Remove nc_check_for_hdf and centralize all file format checking
   NC_check_file_type.
3. Add proper forward search for HDF5 files (but not HDF4 files)
   to look for the magic number at offsets of 0, 512, 1024...
4. Add test tst_hdf5_offset.sh. This tests that hdf5 files with
   an offset are properly recognized. It does so by prefixing
   a legal file with some number of zero bytes: 512, 1024, etc.
5. Off-topic: Added -N flag to ncdump to force a specific output dataset name.