bhyve: support MTU configuration for SLIRP net backend

Support configuring MTU for the SLIRP net backend, for example:

-s 1:0,virtio-net,slirp,mtu=2048,open

Update the manual page accordingly. While here, also document
MAC address configuration.

Reviewed by: markj
Approved by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D54133