• Changed sequence numbers processing to avoid incorrect timeout waiting

when one of links is inactive and have stale sequence number. To avoid
this sequence numbers of all links are getting updated on every
successful packet reassembling.

• ng_ppp_bump_mseq function created to simplify code.
• ng_ppp_frag_drop function separated from ng_ppp_frag_process to

simplify code.

Reviewed by: archie
Approved by: glebius (mentor)