The thredds-test server now has some password protected datasets
that can be used to test DAP2 authorization support.
The general location is
````
https://thredds.ucar.edu/thredds/tdscapabilities/authTest.html
````
and specifically:
````
https://thredds.ucar.edu/thredds/dodsC/test3/testData.nc.html
````

This PR replaces old testcases with ncdap_test/testauth.sh.
This testcase allows us to test use of the .dodsrc file and .netrc file
and embedded user+pwd.

As part of this, I had to create a program (ncdap_test/pathcvt.c)
that is essentially the equivalent to cygpath. Given a path in
windows, unix, msys or cygwin format, it converts it to the
equivalent format in one of those four cases.  So it can be used
to convert a cygwin path to a windows path, for example. This is
needed in testpathcvt and testauth to make sure that the paths
in .daprc (e.g. the reference to .netrc) are of the proper
format.

Misc. Other Changes:
1. Fix some memory leaks in libdap2
2. Setting the env variable CURLOPT_VERBOSE allows tracking of curl
   operations.
3. Make tst_charvlenbug be conditional on NC_VLEN_NOTEST.