canbus: handle socket bind failures

[Opriego]

Summary

• add a focused regression test for CAN socket bind failures
• make canbus_connect return its existing bind error code when bind returns -1

Problem

canbus_connect checks whether bind returns a value less than -1. POSIX
socket errors are reported as -1, so the function ignores a normal bind
failure, continues configuring the socket, marks the client connected, and
returns success.

Fix

Change the bind result check from < -1 to < 0.

Testing

The regression test wraps the socket calls so that socket and ioctl succeed
while bind returns -1. It also verifies that socket options are not applied
after the bind failure.

Before the fix:

0%: Checks: 1, Failures: 1, Errors: 0
canbus_connect(&client) returned 0 instead of 6

After the fix:

100%: Checks: 1, Failures: 0, Errors: 0

The existing canbus_shutdown pointer/integer comparison warning is unrelated
and remains unchanged.