Speedup arbitrary distribution algorithm

Do pointer comparisons rather than strcmps.
~80x speedup
Bug 3529

e.g.
1000 nodes
8000 tasks
[Sep 11 14:24:15.873639 20992 srvcn        0x7f8c1cdda700] _task_layout_hostfile: hostfile processing took usec=2152678 (orig)
[Sep 11 14:27:46.173424 20992 srvcn        0x7f8c1c6d3700] _task_layout_hostfile: hostfile processing took usec=2142997 (orig)
[Sep 11 14:32:32.245420  4037 srvcn        0x7f12de4e4700] _task_layout_hostfile: hostfile processing took usec=26198 (node ptrs)
[Sep 11 14:36:12.88769   4037 srvcn        0x7f12de6e6700] _task_layout_hostfile: hostfile processing took usec=25515 (node ptrs)
[Sep 11 14:41:38.339162  4037 srvcn        0x7f132c8d5700] _task_layout_hostfile: hostfile processing took usec=27459 (node ptrs)
[Sep 11 15:16:59.575189  1874 srvcn        0x7f3dae3f0700] _task_layout_hostfile: hostfile processing took usec=30129 (node ptrs)
[Sep 11 15:20:50.365004  1874 srvcn        0x7f3dc8b34700] _task_layout_hostfile: hostfile processing took usec=29884 (node ptrs)