nsenter: Support specifying namespace by ID

nsenter: Support specifying namespace by ID

Linux kernel v6.19 introduces listns() syscall that lists namespace IDs,
as well as the support for opening an nsfile by a file_handle of ns ID.

This patch allows specifying namespaces by ID for nsenter. For example:

  # nsenter --net=:7

enters init net namespace (7 for NET_NS_INIT_ID).

In the rare case that a ns file name starts with a colon, prepend "./"
for disambiguation.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>