unix: Increase the default datagram recv buffer size

syslog(3) was recently change to support larger messages, up to 8KB.
Our syslogd handles this fine, as it adjusts /dev/log's recv buffer to a
large size. rsyslog, however, uses the system default of 4KB. This
leads to problems since our syslog(3) retries indefinitely when a send()
returns ENOBUFS, but if the message is large enough this will never
succeed.

Increase the default recv buffer size for datagram sockets to support
8KB syslog messages without requiring the logging daemon to adjust its
buffers.

PR: 260126
Reviewed by: asomers
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33380