Werk #16165: check_mk_agent: Set LC_ALL before running the agent

Previously, Checkmk agents would be run with a preset LC_ALL environment variable if neither C.UTF-8 or C.utf-8 locales were installed.

That led to invalid agent output and crashes in section parsing in multiple checks for some of the locales.

Linux, AIX, Solaris, FreeBSD and OpenWrt agents were affected.

Now, LC_ALL variable is set to C for the described case.

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