broker: use libpmi-client

Drop the somewhat contrived boot_pmi.c class from the broker,
and rewrite the PMI bootstrap code using pmi-client.h interfaces
directly.  I think this clarifies the code even though it is quite
verbose.

If PMI doesn't implement pmi_get_id(), derive the session-id from
the "appnum" (numerical jobid).

Don't attempt to call pmi_get_clique_ranks() unless epgm is enabled.

Neither pmi_get_id() nor pmi_get_clique_ranks() are implemented
in the "simple v1" PMI wire protocol, so allowing these functions
to be unimplemented enables Flux to be launched by mpiexec.hydra,
which addresses one goal of flux-framework#398.

src/broker/Makefile.am

@@ -18,8 +18,6 @@ flux_broker_SOURCES = \
  module.h \
  modservice.c \
  modservice.h \
- boot_pmi.c \
- boot_pmi.h \
  overlay.h \
  overlay.c \
  heartbeat.h \