simplify flux-start options

This pull request adds  `flux-start --cmbd-opts opt[,opt...]` which allows arbitrary options to be passed to cmbd.  This allowed `--k-ary, --modules, --modopts` to be removed from flux-start(1).

Then cmbd option changes:
- dropped `--modules name[,name...]`
- added `--module name` and allow it to be used multiple times
- dropped leftover args as module options: `name:key=val name:key=val ...`
- added `--modopt name:key=val` and allow it to b e used multiple times
- added --quiet option per issue #31
- fixed a segfault in short opt processing for -k, -k-ary option

Finally, updated flux-start(1) man page and added cmbd(1) man page.