Allow to only unshare (slapns -U) without creating separate chroot

It is handy to reuse slapns uid setup, because e.g. `unshare -U...` does
not allow to use newuidmap & friends, and so with just unshare it is
hard to create a user namespace where e.g. screen will work (openpty
wants to chown(:tty), gid(tty)=5)

slapns.py

@@ -139,6 +139,10 @@ def idmap_trysetup_viashadow(kind, pid):
 def main():
     slappart = sys.argv[1]
 
+    unshareonly = (slappart == "-U")
+    if unshareonly:
+        slappart = ""
+    if not unshareonly:
         # create directories inside container
         dirv = ["/proc", "/sys",
                 "/bin", "/sbin", "/lib", "/lib64", "/usr/bin", "/usr/lib",
@@ -208,6 +212,7 @@ def main():
     mount("none", slappart + "/tmp", "tmpfs")
     mount("none", slappart + "/run", "tmpfs")
 
+    if not unshareonly:
         # read-only bind mount bin, lib, ... from SR
         # FIXME stub: here we bind from base system for now
         bind("/bin",    slappart + "/bin",  MS_RDONLY)
@@ -235,9 +240,10 @@ def main():
     os.system("/sbin/ifconfig lo 127.0.0.1")    # XXX at least loopback works
 
     # chroot to container
+    if not unshareonly:
         slappart = abspath(slappart)
         os.chdir(slappart)
-    os.chroot(slappart)
+        os.chroot(slappart) # XXX -> try pivot_root
 
     # FIXME stub: -> $SHELL
     os.execv("/bin/bash", ["bash"])