The `gitno` buffer interface is another layer on top of socket reads.
Abstract it a bit into a "static string" that has `git_str` like
semantics but without heap allocation which moves the actual reading
logic into the socket / stream code, and allows for easier future usage
of a static / stack-allocated `git_str`-like interface.

Add functions to use convert a string with length, instead of assuming
NUL termination.

In addition, move the utf8 to 16 conversion routines into the `git_utf8`
namespace instead of using namespaceless `git__` prefixed names.

The `git_fs_path_owner_is_current_user` expects the root dir on unix
(`/`) to be owned by a non-current user. This makes sense unless root
(or euid == 0) is running the tests, which often happens during distro
build / packaging scripts. Allow them to run the tests.

Enforce the RFC for other protocols; Google's questionable choices about
malformed SSH protocols shouldn't impact our ability to properly parse
HTTPS.

Provide our own url parser, so that we can handle Google Code's "fun"
URLs that have a userinfo with an `@` in it. :cry:

The url::scp::invalid_addresses test attempts to test an invalid IPv6
address. It does not, it calls the regular URL parsing function which
treats it like a possibly invalid scheme.

Provide individual file ownership checks for both the current user and
the system user, as well as a combined current user and system user
check.