protosw: retire pr_drain and use EVENTHANDLER(9) directly

The method was called for two different conditions: 1) the VM layer is
low on pages or 2) one of UMA zones of mbuf allocator exhausted.
This change 2) into a new event handler, but all affected network
subsystems modified to subscribe to both, so this change shall not
bring functional changes under different low memory situations.

There were three subsystems still using pr_drain: TCP, SCTP and frag6.
The latter had its protosw entry for the only reason to register its
pr_drain method.

Reviewed by: tuexen, melifaro
Differential revision: https://reviews.freebsd.org/D36164