parser_bison: Introduce tokens for monitor events

parser_bison: Introduce tokens for monitor events

There already is a start condition for "monitor" keyword and also a
DESTROY token. So just add the missing one and get rid of the
intermediate string buffer.

Keep checking the struct monitor::event value in eval phase just to be
on the safe side.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>