Change the default value of the test suite's "--http-library" option back to "neon". Document more carefully the option's meaning.
Change the default value of the test suite's "--http-library" option back to
"neon". Document more carefully the option's meaning.

This reverts a part of the change I made in r39716, for backward compatibility
of the test suite interface, so that it still works e.g. in the build-bot that
builds svn with Neon only and then runs the test suite without specifying the
--http-library option.

* subversion/tests/cmdline/svntest/main.py
  (preferred_http_library): Change the initial value to 'neon'.
  (usage): Clarify the meaning of --http-library in the help message.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@879841 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion