var: add attributes files locations

var: add attributes files locations

Currently, there are some programs which would like to read and parse
the gitattributes files at the global or system levels.  However, it's
not always obvious where these files live, especially for the system
file, which may have been hard-coded at compile time or computed
dynamically based on the runtime prefix.

It's not reasonable to expect all callers of Git to intuitively know
where the Git distributor or user has configured these locations to
be, so add some entries to allow us to determine their location.  Honor
the GIT_ATTR_NOSYSTEM environment variable if one is specified.  Expose
the accessor functions in a way that we can reuse them from within the
var code.

In order to make our paths consistent on Windows and also use the same
form as paths use in "git rev-parse", let's normalize the path before we
return it.  This results in Windows-style paths that use slashes, which
is convenient for making our tests function in a consistent way across
platforms.  Note that this requires that some of our values be freed, so
let's add a flag about whether the value needs to be freed and use it
accordingly.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>