Fix various NTLM/Digest multi-threading and usage issues

- Fix multi-threading bug where NTLM/Digest auth would fail when
concurrent requests were on the wire. The fix for this is too not
internally share QAuthenticationPrivate pointers, since the challenge
values would get overridden in different threads. This was failing
because the internal QAuthenticationPrivate members would have been
set with the status/values of the current request which would mess
up the state of the new request. As currently implemented, the helper
functions inside QAuthenticationPrivate can't call detach to implement
proper copy on write symantics.
- Fix issue where if user was set via constructor, the NTLM domain
parsing would not occur. Parsing of DOMAIN\user is now redone if
proxy type is determined to be NTLM.

Task-number: QTBUG-15472
Task-number: QTBUG-17322
Task-number: QTBUG-18794
Task-number: QTBUG-13063
Task-number: QTBUG-16585
(cherry picked from commit 6ccbfd6ca498da04e4ef02102c4ded9768225b5a)
Cherry-picked-by: Peter Hartmann <phartmann@blackberry.com>

Change-Id: I58ae6ae362baadfb25eaa7b29556e6e8250d5d95
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Doc: remove the duplicate for in QStyle documentation

Task-number: QTBUG-31739

(cherry picked from commit 0a55499c9af9a4b0a98a12a8aba82c3764fa5994)

Change-Id: I1d2d1bb5ca7a19a8c42f11e944e8339e56df6e4b
Reviewed-by: Liang Qi <liang.qi@digia.com>