UDP-Lite: export pcblist via sysctl interface
Export the list of pcbs for UDP-Lite to be consumed by sockstat and
netstat.
Approved by: re (cperciva)
Reviewed by: Peter Lei, Nick Banks
Differential Revision: https://reviews.freebsd.org/D53229
(cherry picked from commit be93b27483680c7e741b1627cc96a131bc99436c)
(cherry picked from commit da7a90f0975030ae05379d59d5a7ec2420dcbd84)