libflux: use internal implementation for flux msg

Problem: The flux_msg_t data structure stores all internal data
within a zmsg_t data structure.  This leads to libflux always
requiring libczmq.

Solution: Remove use of zmsg_t when storing data internally within
flux_msg_t, instead using lists, memory allocations, etc.  Adjust all
API functions accordingly.  In the few functions that still require
zmsg, convert flux_msg_t to/from zmsg_t as needed.  This leads libflux
to still require libczmq for the time being, but decreases the footprint
of functions that need to be migrated out of libflux-core.

Add unit tests as needed to cover new input checks.

src/common/libflux/Makefile.am

@@ -8,6 +8,7 @@ AM_LDFLAGS = \
 AM_CPPFLAGS = \
 	-I$(top_srcdir) \
 	-I$(top_srcdir)/src/include \
+	-I$(top_srcdir)/src/common/libccan \
 	-I$(top_builddir) \
 	-I$(top_builddir)/src/common/libflux \
 	$(JANSSON_CFLAGS) \