expression: print sets and maps in pretty format

expression: print sets and maps in pretty format

Print elements per line instead of all in a single line.
The elements which can be 'short' are printed 5 per line,
and others, like IPv4 addresses are printed 2 per line.

Example:

% nft list ruleset -nnn
table ip t {
	set s {
		type inet_service
		elements = { 1, 2, 3, 4, 10,
			     432, 433, 434, 435, 436,
			     437, 438, 439, 440, 441,
			     442, 443, 444, 445, 446,
			     447, 448, 449, 450, 12345 }
	}

	map m {
		type inet_service . iface_index : verdict
		elements = { 123 . "lo" : accept,
			     1234 . "lo" : accept,
			     12345 . "lo" : accept,
			     12346 . "lo" : accept,
			     12347 . "lo" : accept }
	}

	set s3 {
		type ipv4_addr
		elements = { 1.1.1.1, 2.2.2.2,
			     3.3.3.3 }
	}
}

Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>