[MEDIUM] config: factor out the parsing of 20 req*/rsp* keywords

[MEDIUM] config: factor out the parsing of 20 req*/rsp* keywords

A new function was added to take care of the common code between
all those keywords. This has saved 8 kB of object code and about
500 lines of source code. This has also permitted to spot and fix
minor bugs (allocated args that were never used).

The code could be factored even more but that would make it a bit
more complex which is not interesting at this stage.

Various tests have been performed, and the warnings and errors are
still correctly reported and everything seems to work as expected.