Introduce `git_fs_path`, which operates on generic filesystem paths.
`git_path` will be kept for only git-specific path functionality (for
example, checking for `.git` in a path).

libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

There are a number of tests that modify the global or system
search paths during the tests.  This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.

ld was warning that the size of path changed, rightly so.

Plus a bit of extra paranoia to ensure config object has valid
contents.

When a repository is initialised, we need to probe to see if there is
a global config to load. If this is not the case, the user isn't able
to write to the global config without creating the backend and adding
it themselves, which is inconvenient and overly complex.

Unconditionally create and add a backend for the global config file
regardless of whether it exists as a convenience for users.

To enable this, we allow creating backends to files that do not exist
yet, changing the semantics somewhat, and making some tests invalid.