Add utility/support package for user namespace support

The `pkg/idtools` package supports the creation of user(s) for
retrieving /etc/sub{u,g}id ranges and creation of the UID/GID mappings
provided to clone() to add support for user namespaces in Docker.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

idtools/usergroupadd_unsupported.go

@@ -0,0 +1,12 @@
+// +build !linux
+
+package idtools
+
+import "fmt"
+
+// AddNamespaceRangesUser takes a name and finds an unused uid, gid pair
+// and calls the appropriate helper function to add the group and then
+// the user to the group in /etc/group and /etc/passwd respectively.
+func AddNamespaceRangesUser(name string) (int, int, error) {
+	return -1, -1, fmt.Errorf("No support for adding users or groups on this OS")
+}