`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.