Werk #3650: Fixed bash versionitis, unbreaking cached check results.

Our shebang lines in scripts explicitly state which bash they want, and the bash instances we start from the script should better use the exact same version. If there is a version mismatch, lots of funny things can happen, the most prominent being that caching check results doesn't work anymore (e.g. when using MRPE). This is caused by the bash fixes for the Shellshock vulnerability which change (among other things) how functions are exported to sub-shells. The consequence is that a pre-Shellshock bash doesn't see the functions exported by a post-Shellshock bash, and vice versa. This setup can happen when the shebang and the user's PATH have different ideas about which bash to use.

To fix this, we use the same absolute path for starting the bash sub-processes as the one stated in the corresponding shebang.

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