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
There is no "z" size specifier on MSVC

See http://msdn.microsoft.com/en-us/library/tcxf1dw6.aspx and https://stackoverflow.com/questions/6655410/why-doesnt-zd-printf-format-work-in-vs2010

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Merge pull request #2618 from csware/msvc-no-z-prefix

There is no "z" size specifier on MSVC