Move un-namespaced constant to internal header

FLAG_BITS only seems to be used internally
Don't use cl_git_pass for POSIX functions

If there is a failure then cl_git_pass tries to get the libgit2
error, but p_... functions don't set that.

Also - trailing whitespace cleanup.
Merge pull request #2613 from libgit2/rb/minor-cleanups

Minor cleanups for master