libutil: move xstrsub to libutil/xzmalloc.c
submodule: flux_modfind() search subdirs

Seach each component of the search path recursively.
submodule: send submodule mgmt requests to modules

flux_insmod ("kvs") request is cmb.insmod.
flux_insmod ("sched.backfill") request is sched.insmod,
flux_insmod ("sched.backfill.priority") request is sched.backfill.insmod.

Similar for flux_rmmod.

flux_lsmod now takes a target argument.
If NULL, request is cmb.lsmod.
If "sched", request is sched.lsmod.
submodule: update flux_lsmod usage in modctl

Set target to NULL pending rework of this module.
submodule: install modules to cmb subdir
submodule: unit tests for flux_modfind()

Also remove a stray debugging fprintf
submodule: add some inline docs to module.h
submodule: shorten flux.conf.in module_path

We no longer need to list every module directory, or hardwire
the .libs libtoolism since we search for modules recursively now.
submodule: drop unnecessary include in module.c
Merge pull request #92 from garlick/submodule

generalize module loading for submodules