The mbedtls library uses a callback mechanism to allow downstream users
to plug in their own receive and send functions. We implement `bio_read`
and `bio_write` functions, which simply wrap the `git_stream_read` and
`git_stream_write` functions, respectively.

The problem arises due to the return value of the callback functions:
mbedtls expects us to return an `int` containing the actual number of
bytes that were read or written. But this is in fact completely
misdesigned, as callers are allowed to pass in a buffer with length
`SIZE_MAX`. We thus may be unable to represent the number of bytes
written via the return value.

Fix this by only ever reading or writing at most `INT_MAX` bytes.

The mbedtls stream implementation makes use of some global variables
which are not marked as `static`, even though they're only used in this
compilation unit. Fix this and remove a duplicate declaration.

Our `openssl_write` function calls `SSL_write` by passing in both `data`
and `len` arguments directly. Thing is, our `len` parameter is of type
`size_t` and theirs is of type `int`. We thus need to clamp our length
to be at most `INT_MAX`.

Now that the function `git_stream__write_full` exists and callers of
`git_stream_write` have been adjusted, we can lift logic for short
writes out of the stream implementations. Instead, this is now handled
either by `git_stream__write_full` or by callers of `git_stream_write`
directly.

Similar to the write(3) function, implementations of `git_stream_write`
do not guarantee that all bytes are written. Instead, they return the
number of bytes that actually have been written, which may be smaller
than the total number of bytes. Furthermore, due to an interface design
issue, we cannot ever write more than `SSIZE_MAX` bytes at once, as
otherwise we cannot represent the number of bytes written to the caller.

Unfortunately, no caller of `git_stream_write` ever checks the return
value, except to verify that no error occurred. Due to this, they are
susceptible to the case where only partial data has been written.

Fix this by introducing a new function `git_stream__write_full`. In
contrast to `git_stream_write`, it will always return either success or
failure, without returning the number of bytes written. Thus, it is able
to write all `SIZE_MAX` bytes and loop around `git_stream_write` until
all data has been written. Adjust all callers except the BIO callbacks
in our mbedtls and OpenSSL streams, which already do the right thing and
require the amount of bytes written.

The callback functions that implement the `git_stream` structure are
only used inside of their respective implementation files, but they are
not marked as `static`. Fix this.

Our streams implementation takes a `size_t` that indicates the length of
the data buffer to be written, and returns an `ssize_t` that indicates
the length that _was_ written.  Clearly no such implementation can write
more than `SSIZE_MAX` bytes.  Ensure that each TLS stream implementation
does not try to write more than `SSIZE_MAX` bytes (or smaller; if the
given implementation takes a smaller size).

Avoid the deprecated `git_stream_cb` typedef since we want to compile
the library without deprecated functions or types.  Instead, we can
unroll the alias to its actual type.

Follow up for PR #4917.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

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