Switch from use of LXCPATH to a configurable 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

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>