Move lxc_attach from namespace.c to attach.c and rename it to lxc_attach_to_ns

Move lxc_attach from namespace.c to attach.c and rename it to lxc_attach_to_ns

Since lxc-attach helper functions now have an own source file, lxc_attach is
moved from namespace.c to attach.c and is renamed to lxc_attach_to_ns,
because that better reflects what the function does (attaching to a
container can also contain the setting of the process's personality, adding
it to the corresponding cgroups and dropping specific capabilities).

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>