libflux: add flux_config_load(), flux_config_save()

If config_file argument is NULL, use default ~/.flux/config.
flux-config: add new command

flux-config provides an interface to the flux config file.

This uses libflux/fconfig.[ch], which leverages ZPL format as
used by the zconfig class in czmq.  ZPL is documented in
rfc.zeromq.org/spec:4/ZPL.
flux command: rework environment management

subcommand search path:
   Environment use:    none
   Environment export: none
   Prototype:          <command-line>:<config-file>:<install-path>

module search path
   Environment use:    none
   Environment export: FLUX_MODULE_PATH
   Prototype:          <command-line>:<config-file>:<install-path>

Lua
   Environment use:    LUA_PATH, LUA_CPATH
   Environment export: LUA_PATH, LUA_CPATH
   Prototype:          <command-line>;<config-file>;<environment>;;;

broker path (executable path, not a search path):
   Environment use:    none
   Environment export: FLUX_CMBD_PATH
   Priority order:     1) command line, 2) config file, 3) install-path

The config file priority is
   1) command line, if provided
   2) $(selfdir)/../../flux.conf, if indicated by /proc/self/exe
   3) the default ~/.flux/config, if it exists.
flux command: exec subcommand names with / directly

If a subcommand name includes a "/", try to exec the path directly
without prepending "flux-" and without applying the search path.
For example "flux ./myprotocommand" can be used to run a subcommand
in development.
flux command: export FLUX_CONFIG

Subcommands such as flux-config should pick up the config file
setting used by the flux.  If the default config was used, this
variable is not set.
flux-config: use FLUX_CONFIG config path if set
build: generate flux.conf for in-tree testing
build: build.conf search srcdir for subcommands

Fixes issue #15.
build: fix make dist

The doc/cmd/spellcheck was missing from the dist tarball.
Quiet the build of flux.conf when silent-rules is active.
libflux: fix gcc-4.8.2 compile failure found by Mark

This was mentioned in issue #14
Merge pull request #18 from garlick/flux-config2

Add support for flux-config to manage runtime search paths