Support `infinity` for `StartLimitBurst`

[mdc-webdb]

The documentation:
https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html
says:
'infinity' is an valid value for StartLimitBurst.
But the module reject it with:
parameter 'unit_entry' entry 'StartLimitBurst' expects an Integer value, got String

[bastelfreak]

@mdc-webdb thanks for raising the issue. Are you interested in providing a PR?

[tuxmaster5000]

Yes, send :)