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.

Be more precise about core vs hart

Also fixes #226.

Be more precise about processor vs hart

Fixes #226

Fixes #234.

Fixes #232

Clarify user responsibilities when debugging lr/sc

Modification of > 1024 hart proposal, maintains backwards compatibility

Clarify multiple DMs/harts