Check PHP max_execution_time; increase for one click updates #938

Recent reports of auto-update not completing. Most likely cause is max_execution_time being set too low. (Default on my box is 30s.)

• we know fetchRemoteFile ranges from 6s via curl to 13s via socket
• time how long it currently takes (typically) for unzipping and copying
• add system check and warning to installer
• one click update: warn if set too low; try to increase limit

I think it's fine to set it to unlimited during the one click update. This could be a static function in Piwik:: that we could also call from the archiving code (maybe this is already done?)

In [1407], refs #938 - add Piwik::setMaxExecutionTime()

In [1411], refs #938 - replace set_time_limit() with call to Piwik::setMaxExecutionTime().

In [1408#file3], call Piwik::setMaxExecutionTime(0) for one click update and command line database update.

(For archiving, defer to LaunchArchiving plugin.)