Add configurations from libpod.conf to containers.conf.Use merge code from libpod to read and merge configurations from file.
Add unshare_linux for build linux: HomeDir should buildah not only for linux. so add unshare_linux for linux and unshare.go for other
convert bool to optinalbool:Use OptionalBool for on-disk configurations so we can distinguish field is undefined vs. user set to false.
Signed-off-by: Qi Wang <qiwan@redhat.com>
Qi Wang