Move to the `git_error` name in the internal API for error-related
functions.

Previously, an error in `git_config_next` would be mistaken as a
successful load, because the previous call would have succeeded.
Coverity saw the subsequent check for a completed iteration as dead, so
let's make it useful again.

CID 1391374

Use the new object_type enumeration names within the codebase.

Use the new-style index names throughout our own codebase.

Instead of using the `khiter_t`, `git_strmap_iter` and `khint_t` types,
simply use `size_t` instead. This decouples code from the khash stuff
and makes it possible to move the khash includes into the implementation
files.

The submodule code currently has its own implementation of a string map,
which overrides the hashing and hash equals functions with functions
that ignore potential trailing slashes. These functions aren't actually
used by our code, making them useless.

These can be used to inject options in an implementation which performs a
recursive clone by executing an external command via crafted url and path
attributes such that it triggers a local executable to be run.

The library is not vulnerable as we do not rely on external executables but a
user of the library might be relying on that so we add this protection.

This matches this aspect of git's fix for CVE-2018-17456.

The header "config_file.h" has a list of inline-functions to access the
contents of a config backend without directly messing with the struct's
function pointers. While all these functions are called
"git_config_file_*", they are in fact completely backend-agnostic and
don't care whether it is a file or not. Rename all the function to
instead be backend-agnostic versions called "git_config_backend_*" and
rename the header to match.

When populating the list of submodule names, we use the submodule
configuration entry's name as the key in the map of submodule names.
This creates a hidden dependency on the liveliness of the configuration
that was used to parse the submodule, which is fragile and unexpected.

Fix the issue by duplicating the string before writing it into the
submodule name map.

Reported by Coverity, CID 1393237