libct/userns: change RunningInUserNS to a wrapper instead of an alias

This was a poor decision on my side; 4316df8b53
moved this utility to a separate package, and split the exported function
from the implementation (and stubs). Out of convenience, I used an alias
for the latter part, but there's two downsides to that;

- `RunningInUserNS` being an exported var means that (technically) it can
  be replaced by other code; perhaps that's a "feature", but not one we
  intended it to be used for.
- `RunningInUserNS` being implemented through a var / alias means it's
  also documented as such on [pkg.go.dev], which is confusing.

This patch changes it to a regular function, acting as a wrapper for
the underlying implementations. While at it, also slightly touching
up the GoDoc to describe its functionality / behavior.

[pkg.go.dev]: https://pkg.go.dev/github.com/opencontainers/runc@v1.1.13/libcontainer/userns#RunningInUserNS

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libcontainer/userns/userns.go

@@ -1,4 +1,8 @@
 package userns
 
-// RunningInUserNS detects whether we are currently running in a user namespace.
-var RunningInUserNS = runningInUserNS
+// RunningInUserNS detects whether we are currently running in a Linux
+// user namespace and memoizes the result. It returns false on non-Linux
+// platforms.
+func RunningInUserNS() bool {
+	return runningInUserNS()
+}