MFC r316743:

The sysctl variable net.inet.tcp.drop_synfin is not honored in all states,
for example not in SYN-SENT.
This patch adds code to check the sysctl variable in other states than
LISTEN.
Thanks to ae and gnn for providing comments.

Reviewed by: gnn
Differential Revision: https://reviews.freebsd.org/D9894