Streamline PHP environment setting values. The PHP
environment setting `max_execution_time` is an integer
according to the PHP manual. This must be reflected in
the INSTALL.md. Furthermore, it can be confusing for
beginners.

Resolves: #79607
Releases: master
Change-Id: I79f1bd6386b1ebe0409b02528373b10f42e57847
Reviewed-on: https://review.typo3.org/51513


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Tested-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>