sysutils/conmon: Use send(2) for sequenced packet sockets

Podman container stdout and stderr are proxied through conmon, allowing
logging and supporting attaching/detaching from the container. The two
output streams are multiplexed on a single SOCK_SEQPACKET unix domain
socket. Each packet contains either stdout or stderr data with a small
header identifying the stream.

On FreeBSD versions prior to FreeBSD-15.0-RELEASE, the write(2) system
call implicitly marks each write as a single packet on the stream but in
15.0 and later, this is not the case - packet boundaries must be
explicitly marked using send(2). This resulted multiple writes being
combined in a single packet and caused Podman to exit with an error
(https://github.com/containers/podman/issues/27918).

This changes conmon to use send(2) to write the output packets which is
compatible with all FreeBSD versions and requires no changes to the
Podman side of the connection.