Make config api items const

Make config api items const

This makes it easier to write a binding, and presents a cleaner API. Use
strdupa in a few places to get mutable strings for tokenizing / parsing.
Also change the argv type in lxcapi_start and lxcapi_create to match
that of execv(3).

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>