Lock  m_critsect in POSIX wxThread::Create().

This is the documented behavior: all methods accessing m_internal should
lock it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73997 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Move POSIX implementation of wxThread::Create() to wxThreadInternal.

No real changes, just refactoring to follow the same pattern as other
implementations use.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73998 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Call wxThread::Create() from Run() automatically.

In the common case, when Run() is called immediately after Create() and
default stack size is used, it's unnecessarily verbose.  Just create the
thread in Run() if it wasn't done explicitly yet.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73999 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775