lxc-busybox: Prevent copying binaries from /usr/local to container

lxc-busybox: Prevent copying binaries from /usr/local to container

On certain systems, some binaries needed by the container features (dropbear,
openssh), may be placed in non-standard (aka non-distribution-managed
locations), such as /usr/local/*, /opt/local/*, etc. Don't copy the respective
binaries in the container and return a clear error why.

The user should only use these binaries if they are installed at system-wide
locations on the host, such as /{s,}bin or /usr/{s,}bin.

v2:
- check that binary paths adhere to /{,usr/}{,s}bin only

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>