The overflow check in `read_reuc` tries to verify if the
`git__strtol32` parses an integer bigger than UINT_MAX. The `tmp`
variable is casted to an unsigned int for this and then checked
for being greater than UINT_MAX, which obviously can never be
true.

Fix this by instead fixing the `mode` field's size in `struct
git_index_reuc_entry` to `uint32_t`. We can now parse the int
with `git__strtol64`, which can never return a value bigger than
`UINT32_MAX`, and additionally checking if the returned value is
smaller than zero.

We do not need to handle overflows explicitly here, as
`git__strtol64` returns an error when the returned value would
overflow.

This allows the application to use their own TLS stream, regardless of
the capabilities of libgit2 itself.

We should explicitly include the declaration of git_strarray
from "include/git2/sys/transport.h"

As refdb and odb backends can be allocated by client code, libgit2
can’t know whether an alternative memory allocator was used, and thus
should not try to call `git__free` on those objects.

Instead, odb and refdb backend implementations must always provide
their own `free` functions to ensure memory gets freed correctly.

libgit2 implementations of smart subtransports can simply reach through
the structure, but external implementors cannot.

Add these two functions as a way for the smart subtransports to get the
callbacks as set by the user.

When a configuration file is locked, any updates made to it will be done
to the in-memory copy of the file. This allows for multiple updates to
happen while we hold the lock, preventing races during complex
config-file manipulation.

Resolve documentation warnings