src: add and use `set_is_meter` helper

src: add and use `set_is_meter` helper

The sets constructed for meters are flagged as anonymous and dynamic.
However, in some places there are only checks that they are dynamic,
which can lead to normal sets being classified as meters.

For example:

  # nft add table t
  # nft add set t s { type ipv4_addr; size 256; flags dynamic,timeout; }
  # nft add chain t c
  # nft add rule t c tcp dport 80 meter m size 128 { ip saddr limit rate 10/second }
  # nft list meters
  table ip t {
          set s {
                  type ipv4_addr
                  size 256
                  flags dynamic,timeout
          }
          meter m {
                  type ipv4_addr
                  size 128
                  flags dynamic
          }
  }
  # nft list meter t m
  table ip t {
          meter m {
                  type ipv4_addr
                  size 128
                  flags dynamic
          }
  }
  # nft list meter t s
  Error: No such file or directory
  list meter t s
               ^

Add a new helper `set_is_meter` and use it wherever there are checks for
meters.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>