when log_tag is given in spawn/run, send to syslog

Add syslog_socket configuration, which determines where these are
directed to. If not specified (nil), it uses the default from the
`logger` command (probably /dev/log).

This can be used to efficiently emit logs from a container without
having to go through the Warden server.

[#61998582]

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>