Refactor locks package to build on non-Linux

Move SHM specific code into a subpackage. Within the main locks
package, move the manager to be linux-only and add a non-Linux
unsupported build file.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

libpod/lock/shm_lock_manager_unsupported.go

@@ -0,0 +1,29 @@
+// +build !linux
+
+package lock
+
+import "fmt"
+
+// SHMLockManager is a shared memory lock manager.
+// It is not supported on non-Unix platforms.
+type SHMLockManager struct{}
+
+// NewSHMLockManager is not supported on this platform
+func NewSHMLockManager(numLocks uint32) (Manager, error) {
+	return nil, fmt.Errorf("not supported")
+}
+
+// OpenSHMLockManager is not supported on this platform
+func OpenSHMLockManager(numLocks uint32) (Manager, error) {
+	return nil, fmt.Errorf("not supported")
+}
+
+// AllocateLock is not supported on this platform
+func (m *SHMLockManager) AllocateLock() (Locker, error) {
+	return nil, fmt.Errorf("not supported")
+}
+
+// RetrieveLock is not supported on this platform
+func (m *SHMLockManager) RetrieveLock(id string) (Locker, error) {
+	return nil, fmt.Errorf("not supported")
+}