Remove a bashism in ocf-shellfuncs.in

The function ocf_bash_has_xtracefd and its alias ocf_is_bash4 break non bash shells (in my case dash) because of how bash version is tested. Due to local environment idiosyncrasies it is perfectly possible to have dash running a script while the variable SHELL has a value of /bin/bash. It works fine for most of the code except the tracing functionality.

What I propose is to fix the version detection mechanism by first checking against the usually not exported variable BASH_VERSION.