Make software iSCSI more configurable.

Move software iSCSI tunables/sysctls into kern.icl.soft subtree.
Replace several hardcoded length constants there with variables.

While there, stretch the limits to better match Linux' open-iscsi
and our own initiator with new MAXPHYS of 1MB. Our CTL target is
also optimized for up to 1MB I/Os, so there is also a match now.
For Windows 10 and VMware 6.7 initiators at default settings it
should make no change, since previous limits were sufficient there.

Tests of QD1 1MB writes from FreeBSD over 10GigE link show throughput
increase by 29% on idle connection and 132% with concurrent QD8 reads.

MFC after: 3 days
Sponsored by: iXsystems, Inc.

(cherry picked from commit b75168ed24ca74f65929e5c57d4fed5f0ab08f2a)