osutil: add function to detect merged /usr systems (MR 39)

This is very crude, but in lieu of an actual spec for usr merge or some
standard way to detect whether it's done, this will have to do for now.
We can and should improve it later!

internal/osutil/osutil.go

@@ -10,6 +10,23 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// Try to guess whether the system has merged dirs under /usr
+func HasMergedUsr() bool {
+	for _, dir := range []string{"/bin", "/lib"} {
+		stat, err := os.Lstat(dir)
+		if err != nil {
+			// TODO: probably because the dir doesn't exist... so
+			// should we assume that it's because the system has some weird
+			// implementation of "merge /usr"?
+			return true
+		} else if stat.Mode()&os.ModeSymlink == 0 {
+			// Not a symlink, so must not be merged /usr
+			return false
+		}
+	}
+	return true
+}
+
 // Converts a relative symlink target path (e.g. ../../lib/foo.so), that is
 // absolute path
 func RelativeSymlinkTargetToDir(symPath string, dir string) (string, error) {