Allow additional arguments to be passed into hooks

If a packager wants to be able to support addititional arguments on his
hook this will allow them to setup the configuration with these arguments.

For example this would allow a hook developer to add support for a --debug
flag to change the level of debugging in his hook.

In order to complete this task, I had to vendor in the latest
github.com://opencontainers/runtime-tools, which caused me to have to fix a
Mount and Capability interface calls

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

vendor/github.com/opencontainers/runtime-tools/filepath/separator.go

@@ -0,0 +1,9 @@
+package filepath
+
+// Separator is an explicit-OS version of path/filepath's Separator.
+func Separator(os string) rune {
+	if os == "windows" {
+		return '\\'
+	}
+	return '/'
+}