Switch from use of LXCPATH to a configurable default_lxc_path

Switch from use of LXCPATH to a configurable default_lxc_path

Here is a patch to introduce a configurable system-wide
lxcpath.  It seems to work with lxc-create, lxc-start,
and basic python3 lxc usage through the api.

For shell functions, a new /usr/share/lxc/lxc.functions is
introduced which sets some of the basic global variables,
including evaluating the right place for lxc_path.

I have not converted any of the other python code, as I was
not sure where we should keep the common functions (i.e.
for now just default_lxc_path()).

configure.ac: add an option for setting the global config file name.
utils: add a default_lxc_path() function
Use default_lxc_path in .c files
define get_lxc_path() and set_lxc_path() in C api
use get_lxc_path() in lua api
create sh helper for getting default path from config file
fix up scripts to use lxc.functions

Changelog:
  feb6:
	fix lxc_path in lxc.functions
	utils.c: as Dwight pointed out, don't close a NULL fin.
	utils.c: fix the parsing of lxcpath line
	lxc-start: print which rcfile we are using
	commands.c: As Dwight alluded to, the sockname handling was just
	   ridiculous.  Clean that up.
	use Dwight's recommendation for lxc.functions path: $datadir/lxc
	make lxccontainer->get_config_path() return const char *
		Per Dwight's suggestion, much nicer than returning strdup.
  feb6 (v2):
        lxccontainer: set c->config_path before using it.
	convert legacy lxc-ls

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>