canbus: honor shutdown mode

[Opriego]

Summary

• add a focused regression test for canbus_shutdown
• forward the caller-provided shutdown mode to the socket syscall

Problem

canbus_shutdown accepts a how argument but ignores it and always calls
shutdown with SHUT_RD. Requests such as SHUT_WR or SHUT_RDWR therefore
have the wrong behavior.

Fix

Pass how to shutdown instead of the hard-coded SHUT_RD value.

Testing

The regression test wraps the socket syscall, calls canbus_shutdown with
SHUT_WR, and records the mode received by shutdown.

Before the fix:

0%: Checks: 1, Failures: 1, Errors: 0
shutdown received SHUT_RD (0) instead of SHUT_WR (1)

After the fix:

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

The existing socket descriptor comparison against NULL is unrelated and
remains unchanged.