BUG/MINOR: config: insufficient syntax check of the global "maxconn" value

BUG/MINOR: config: insufficient syntax check of the global "maxconn" value

The maxconn value is decoded using atol(), so values like "3k" are
rightly converter as interger 3, while the user wants 3000.

This patch fixes this behavior by reporting a parsing error.

This patch could be backported on all maintained version, but it
could break some configuration. The bug is really minor, I recommend
to not backport, or backport a patch which only throws a warning in
place of a fatal error.