Clarify triggers

Remove implemented features from Future Ideas.

NMI Clarification

Clarify SBA unsupport access size error.

Maybe solves #252.

Clarify high bits of sbdata in narrow reads.

Explain why progbufsize=1 is special

hawindowsel can be smaller, depends on # of harts

Nonexistent/unavailable harts are not halted.

Clarify abstract command errors.

haltsum1: correct its address 

travis: add 'make chisel' target to regression

Change dcsr.prv reset value to 3

This will help avoid a trap where an implementation might end up with
prv 0 when entering debug out of reset. Even in that case prv should be
set from the privilege mode of the processor, which should be machine
mode, but specifying this as the actual value makes things a little
simpler.