Improve readability of execute functions

[ezickler]

This is just an idea to improve the readability and thus make it easy for new devs to understand the code.

I noticed that in jkv, jobject and jdistributed-object many but not all exec function have a repeated pattern of checking for local backend existence. It is check inconsistently either via != or ==. Doing this in a consistent way would make it easier to read and understand.

Here is an example:
https://github.com/wr-hamburg/julea/blob/879b745059b41abd76fa816bd9458d49dcd89441/lib/object/jobject.c#L188-L228

[michaelkuhn]

The checks should be more consistent in b14074e.