Update pfkey_open() function to set socket's write buffer size to
128k and receive buffer size to 2MB. In case if system has bigger
default values, do not lower them.

This should partially solve the problem, when setkey(8) returns
EAGAIN error on systems with many SAs or SPs.

PR:		[[ https://bugs.freebsd.org/88336 | 88336 ]]
Obtained from:	NetBSD/ipsec-tools