Fix problems in settings parser

There were 2 problems with the settings parser, that this fixes:
1. extruder positions for extruder 4-15 and 0 could not be parsed, but you allow 16 extruders (which Repetier-Host also uses,also most likely as virtual extruder for mixing systems). Now these settings get read.
2. An empty multiline parameter caused an error. This happens if you have e.g. no extruder switch scripts. Now there is a test for multiline parameter and empty multiline parameter do not trigger an error message any more.