implement lxc_string_split_quoted

implement lxc_string_split_quoted

lxc_string_split_quoted() splits a string on spaces, but keeps
groups in single or double qoutes together.  In other words,
generally what we'd want for argv behavior.

Switch lxc-execute to use this for lxc.execute.cmd.

Switch lxc-oci template to put the lxc.execute.cmd inside single
quotes, because parse_line() will eat those.  If we don't do that,
then if we have lxc.execute.cmd = /bin/echo "hello, world", then the
last double quote will disappear.

Signed-off-by: Serge Hallyn <shallyn@cisco.com>