Werk #3234: Make unixcat more robust

Component	Core & setup
Title	Make unixcat more robust
Date	Feb 23, 2016
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.4.0i1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.2.8b3 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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).

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