MFC r340135: Make ng_pptpgre(8) netgraph node be able to restore order

MFC r340135: Make ng_pptpgre(8) netgraph node be able to restore order
for packets reordered in transit instead of dropping them altogether.
It uses sequence numbers of PPtPGRE packets.

A set of new sysctl(8) added to control this ability or disable it:

net.graph.pptpgre.reorder_max (1) defines maximum length of node's
private reorder queue used to keep data waiting for late packets.
Zero value disables reordering. Default value 1 allows the node to restore
the order for two packets swapped in transit. Greater values allow the node
to deliver packets being late after more packets in sequence
at cost of increased kernel memory usage.

net.graph.pptpgre.reorder_timeout (1) defines time value in miliseconds
used to wait for late packets. It may be useful to increase this
if reordering spot is distant.