We use get_opt_long, which allows non ambiguous abbreviations, but
since we didn't like that we have code that checks for abbreviations and
issues an error. But that code only handled separate argument and key like:
--bla value
and didn't handle them in the same arguemnts, like:
--bla=value
This patch fixes that, and gives a slightly better error report (it still
contains the =value part though)
