flux-module: display dlerror() on failure

We had a silent exit(1) on flux module load by path where
the file exists but dlopen()/dlsym() fails.  dlopen() fails
when there is any unresolved symbol, so expect this to happen.
build: fix typo in pkg-config file

Typo was preventing flux-core.pc from working properly when installed
to a prefix in home directory.
build: include flux_barrier() in -lflux-core
Merge pull request #9 from garlick/misc-fixes

Misc fixes