evaluate: display expression, statement and command name on debug

evaluate: display expression, statement and command name on debug

Extend debugging knob for evaluation to display the command, the
expression and statement names.

 # nft --debug=eval add rule x y ip saddr 1.1.1.1 counter
 <cmdline>:1:1-37: Evaluate add
 add rule x y ip saddr 1.1.1.1 counter
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 <cmdline>:1:14-29: Evaluate expression
 add rule x y ip saddr 1.1.1.1 counter
              ^^^^^^^^^^^^^^^^
 ip saddr $1.1.1.1

 <cmdline>:1:14-29: Evaluate relational
 add rule x y ip saddr 1.1.1.1 counter
              ^^^^^^^^^^^^^^^^
 ip saddr $1.1.1.1

 <cmdline>:1:14-21: Evaluate payload
 add rule x y ip saddr 1.1.1.1 counter
              ^^^^^^^^
 ip saddr

 <cmdline>:1:23-29: Evaluate symbol
 add rule x y ip saddr 1.1.1.1 counter
                       ^^^^^^^

 <cmdline>:1:23-29: Evaluate value
 add rule x y ip saddr 1.1.1.1 counter
                       ^^^^^^^
 1.1.1.1

 <cmdline>:1:31-37: Evaluate counter
 add rule x y ip saddr 1.1.1.1 counter
                               ^^^^^^^
 counter packets 0 bytes 0

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>