cmd/upspin: add -L (follow links) and -R (recursive) flags to ls
cmd/upspin: add -L (follow links) and -R (recursive) flags to ls

Similar to the same flags in Unix ls.

Change-Id: Ieff16ac66faf10402d964457f2a67743b60d24cf
Reviewed-on: https://upspin-review.googlesource.com/4780
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin