Fix build.
counter.h requires systm.h
Fix build.
Use an unsigned long when indexing into mfchashtbl[] and mf6ctable[].  This
matches the types used when computing hash indices and the type of the
maximum size of mfchashtbl[].

PR:		kern/181821
Submitted by:	Sven-Thorsten Dietrich <sven@vyatta.com> (IPv4)
MFC after:	1 week
Use LIST_FOREACH_SAFE() instead of doing it by hand.