ENH(BF?): overload open() (for buffering) within filtertestcase to guarantee atomic writing

This is with the hope to further resolve random tests failures
( primarily on fast travis-ci systems ;) )
TST: be more aggressive in cleanup of temp files + use mktemp instead of mkstemp
DOC: minor change -- refer to the fail2ban manpage
ENH: deleted trailing spaces in fail2ban- cmdline tools

Now it was already a mix, and Cyril is not working on this code any
longer so no need to maintain this convention.