cmds: Fix the PUTVAL command.

cmds: Fix the PUTVAL command.

This is a partial roll-back of previous code that tried to convert PUTVAL
to use metric_t. This role is now filled by PUTMETRIC instead.

Modifications to the "common" plugin (particularly "parse_identifier" and
"parse_identifier_vl" fix the "cmds" unit tests.

The "cmd_create_putval" function has been removed because output will use
the new metric format instead.