virt: Detect proot virtualisation by ptrace metadata

proot provides userspace-powered emulation of chroot and mount --bind,
lending it to be used on environments without unprivileged user
namespaces, or in otherwise restricted environments like Android.

In order to achieve this, proot makes use of the kernel's ptrace()
facility, which we can use in order to detect its presence. Since it
doesn't use any kind of namespacing, including PID namespacing, we don't
need to do any tricks when trying to get the tracer's metadata.

For our purposes, proot is listed as a "container", since we mostly use
this also as the bucket for non-container-but-container-like
technologies like WSL. As such, it seems like a good fit for this
section as well.

man/systemd-detect-virt.xml

@@ -167,6 +167,11 @@
             <entry><varname>wsl</varname></entry>
             <entry><ulink url="https://docs.microsoft.com/en-us/windows/wsl/about">Windows Subsystem for Linux</ulink></entry>
           </row>
+
+          <row>
+            <entry><varname>proot</varname></entry>
+            <entry><ulink url="https://proot-me.github.io/">proot</ulink> userspace chroot/bind mount emulation</entry>
+          </row>
         </tbody>
       </tgroup>
     </table>