Want updates for MDB module API

[melloc]

Beyond the functions mentioned in #50 for tab completion, other undocumented functions are:

• mdb_aread() / mdb_awrite() (added in illumos#3946)
• mdb_symbol_iter() (added in illumos#3985; a number of new #defines were added for this, and should also be documented)
• mdb_getareg() / mdb_object_iter() (added in 6935550)
• mdb_ffs() (added in 6456379, to provide an ffs(3C)-like mechanism)
• mdb_get_state(), which returns one of:
  • MDB_STATE_IDLE, target is idle (not running yet)
  • MDB_STATE_RUNNING, target is currently executing
  • MDB_STATE_STOPPED, Target is stopped
  • MDB_STATE_UNDEAD, Target is undead (zombie)
  • MDB_STATE_DEAD, target is dead (core dump)
  • MDB_STATE_LOST, target lost by debugger
• mdb_callback_add() / mdb_callback_remove(), which allow adding and removing callbacks, which will be called either:
  • When the target has changed state after continuing (MDB_CALLBACK_STCHG)
  • Just before printing the prompt (MDB_CALLBACK_PROMPT)

Additionally, there are two exported symbols that should probably be documented:

• mdb_prop_kernel, "are we looking at a kernel?"
• mdb_prop_postmortem, "are we looking at a static dump?"