Flip THREADSAFE's default to ON

Threads are here to stay; and for a while now, users have had to call
the initialization function which sets up threads and crypto regardless
of whether the library was built threadsafe or not.
Merge pull request #2666 from libgit2/cmn/threadsafe-on

Flip THREADSAFE's default to ON
Removed some useless variable assignments
Fixed potential crash with uninitialized variables
Merge pull request #2659 from swisspol/static_analysis

Fixed the most obvious Clang static analyzer warnings
revert tests: always use core.autocrlf=false in tests
Merge pull request #2669 from ethomson/revert_test_fix

revert tests: always use core.autocrlf=false in tests
Rename git_threads_ to git_libgit2_

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.