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

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

The underlying code uses GIT_CONFIG_FILENAME_GLOBAL, which is .gitconfig.

In order to reject writes to included configuration entries, we need to
keep track of whether an entry was included via another configuration
file or not. This information is being stored in the `cvar` structure,
which is a rather weird location, as it is only used to create a list
structure of config entries.

Move the include depth into the structure `git_config_entry` instead.
While this fixes the layering issue, it enables users of libgit2 to
access the depth, too.

config: specify how we match the regular expressions

We do it the same as git does: case-sensitively on the normalized form of the
variable name.

While here also specify that we're case-sensitive on the values when handling
the values when setting or deleting multivars.

Our current configuration logic is completely oblivious of any
repository, but only cares for actual file paths. Unfortunately, we are
forced to break this assumption by the introduction of conditional
includes, which are evaluated in the context of a repository. Right now,
only one conditional exists with "gitdir:" -- it will only include the
configuration if the current repository's git directory matches the
value passed to "gitdir:".

To support these conditionals, we have to break our API and make the
repository available when opening a configuration file. This commit
extends the `open` call of configuration backends to include another
repository and adjusts existing code to have it available. This includes
the user-visible functions `git_config_add_file_ondisk` and
`git_config_add_backend`.

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.