libcontainer/configs is used by the docker user namespace proposed
patchset to use IDMap for uid/gid maps across the codebase. Given the
client uses some of this code, it needs to build on non-Linux. This
separates out the Linux-only syscalls using build tags.
Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
Phil Estes