The prototypes never change and they are called only from main.cc.

When the idle proc blocks while executing some work, another proc
takes over the idling.  This implementation is optimized for
non-blocking work.

(reproducing mapbench behavior in userspace)

memory allocators.  this fixes a bug that caused gdb to crash..

now utest gets good scalability for random add/remove operations on a
1024-element crange:

nickolai@ben:~/proj/xv6$ for n in 1 2 4 8 16 32 64 80; do ( time ./o.user/utest -n $n ) 2>&1 | grep ^real | awk '{print "'$n'", $2;}'; done
1 0m21.570s
2 0m16.342s
4 0m9.997s
8 0m5.484s
16 0m3.218s
32 0m1.978s
64 0m1.741s
80 0m1.814s
nickolai@ben:~/proj/xv6$

get rid of __sync_synchronize in gc_begin_epoch, since a LOCK instruction
acts as an mfence.

useful ref: http://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf

Otherwise we leave junk dirents on unlink.