Add -gc-start-position and -gc-end-position options for bchscheme.

Add -gc-start-position and -gc-end-position options for bchscheme.
These options delimit the region of the gc file where bchscheme will
write, thus two bchschemes can share the same file.  The region is
locked using lockf.

Change sleep_on_pid to sleep_awaiting_drones that uses a bit mask
specifying which drones we are waiting for.

Reorder some operations to reduce the size of some potential interrupt
windows.