detect-virt: add --private-users switch to check if a userns is active

Various things don't work when we're running in a user namespace, but it's
pretty hard to reliably detect if that is true.

A function is added which looks at /proc/self/uid_map and returns false
if the default "0 0 UINT32_MAX" is found, and true if it finds anything else.
This misses the case where an 1:1 mapping with the full range was used, but
I don't know how to distinguish this case.

'systemd-detect-virt --private-users' is very similar to
'systemd-detect-virt --chroot', but we check for a user namespace instead.

src/basic/virt.h

@@ -67,6 +67,7 @@ int detect_vm(void);
 int detect_container(void);
 int detect_virtualization(void);
 
+int running_in_userns(void);
 int running_in_chroot(void);
 
 const char *virtualization_to_string(int v) _const_;