more vzlogger.conf validations

[andig]

Follow-up to #253: additional validations and warning messages:

• add warning if aggtime is set without aggmode (any meter type). If we want to get fancy we could determine aggmode from middleware (probably feature for Validate channel configuration against middleware #257).
• add warning for S0 meters when addmode != sum. This is more specific for the often-used S0 meters.
• improve error message- when parsing fails output link to config editor

[r00t-]

add warning for S0 meters when addmode != sum

for the impulse channel, NOT for the power channel!

[r00t-]

also:
add warning if aggtime < interval (part of #433 )