[FEATURE] Only allow access to install tool for system maintainers

A new option $GLOBALS['TYPO3_CONF_VARS']['SYS']['systemMaintainers']
is introduced, which contains a list of Backend User uids. It is
then possible to restrict access to backend modules to system
maintainers - most importantly the four Install Tool modules.

When this option is not set in LocalConfiguration.php, then all
admins are system maintainers, same goes for accessing TYPO3
in Development context.

This is the first step to remove the necessary "enter your
intall tool password" when accessing the install tool from
within TYPO3 Backend.

Resolves: #82266
Releases: master
Change-Id: If0026748ede694394b4e7f39788a650816160146
Reviewed-on: https://review.typo3.org/53830


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>