Added support for extra ifconfig args to jail ip4.addr & ip6.addr params

This allows for CARP interfaces to be  used in jails e.g.
ip4.addr = "em0|10.10.1.20/32 vhid 1 pass MyPass advskew 100"

Before this change using exec.prestart to configure a CARP address
would result in the wrong MAC being broadcast on startup as jail creates
IP aliases to support ip[4|6].addr before exec.prestart is executed.

PR:		191832
Reviewed by:	jamie
MFC after:	1 week
X-MFC-With:	r269340
Phabric:	D528
Sponsored by:	Multiplay
set EXT2_LINK_MAX to LINK_MAX

In linux EXT4_LINK_MAX is now 64000.  We can't really do that
since i_nlink and va_nlink are signed so setting higher values
is likely to cause trouble.

This is a system limitation so set the EXT_LINK_MAX to
what the system can handle.

MFC after:	3 days