BUG/MINOR: cfgparse-listen: always reject extra arguments to "persist"

BUG/MINOR: cfgparse-listen: always reject extra arguments to "persist"

The check for extra arguments was placed inside the "if" block which
parses "persist rdp-cookie(<name>)", so "persist rdp-cookie foo" was
silently accepted. Let's move it to the beginning of the keyword's
processing so that it covers all the cases.

This must NOT be backported since it makes the config parser stricter.