Fixes issues with coins that did not implement a proper array index into
their `getmininginfo` output. Currently supported keys are
* networkhashps
* hashespersec
* netmhashps
If none of there are found or `getmininginfo` fails itself, we now
default to `0` to ensure the webfrontend still works.
Fixes#847 and also fixes#834 which has not reported back the
required information.
d95961b470