syslogd: Open forwarding socket descriptors

Previously, when forwarding a message to a remote address, the target's
addrinfo was saved at config-parse-time. When message-deliver-time came,
the message's addrinfo was passed into sendmsg(2) and delivered by the
first available inet socket.

Passing an addrinfo into sendmsg(2) is prohibited in Capsicum capability
mode, so sockets are now opened and connected to their remote peers at
config-parse-time when executing outside of the capability sandbox.

These connected socket descriptors are saved and passed into sendmsg(2),
allowing forwarding to be performed inside of the capability sandbox.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D47104