uid-range: add new uid_range_load_userns_by_fd() helper

This is similar to uid_range_load_userns() but instead of reading the
uid_map off a process it reads it off a userns fd.

(Of course the kernel has no API for this right now, hence we fork off a
throw-away process which joins the user namespace, and then read off the
data from there.)

src/basic/uid-range.h

@@ -43,5 +43,6 @@ typedef enum UIDRangeUsernsMode {
 } UIDRangeUsernsMode;
 
 int uid_range_load_userns(UIDRange **ret, const char *path, UIDRangeUsernsMode mode);
+int uid_range_load_userns_by_fd(int userns_fd, UIDRangeUsernsMode mode, UIDRange **ret);
 
 bool uid_range_overlaps(const UIDRange *range, uid_t start, uid_t nr);