pflog: use nd_ types in struct pfloghdr.

pflog: use nd_ types in struct pfloghdr.

This 1) makes sure that GET_ macros are used to extract data from the
structure (which they already were) and 2) avoids undefined behavior if
the structure isn't aligned on the appropriate memory boundary.

Fixes #1054.  (The SNMP issues are fixed by changes for #1012.)

(cherry picked from commit a0b785957c77e014b566949bf00ff746e2071813)