RELENG-183: restore use of 'mk-agents-shar' and other tweaks to fix 'gmake agentsshar' (don't rebuild unnecessarily, assert expected bits are built)
RELENG-183: restore use of 'mk-agents-shar' and other tweaks to fix 'gmake agentsshar' (don't rebuild unnecessarily, assert expected bits are built)

joyent/mountain-gorilla,joyent/mountain-gorilla,joyent/mountain-gorilla