Check_MK Werk 3234: Make unixcat more robust

ID: 3234 Title: Make unixcat more robust Component: Core & Setup Level: 1 Class: Bug Fix Version: 1.2.9i1 Previously it could happen that unixcat crashed when system resources and/or limits were low, especially the nproc limit. Now we emit sensible error messages and exit with new return codes (5 and 6).