This test is done quite a bit across the tree, and the open-coded variants make it easy to have an
accidental mismatch between the length of the prefix being tested and the length actually passed to
strncmp().
This fixes an issue of that type comparing the server version against the prefix "u2.10", where the old
code used an incorrect length of 4.
As always, the funcion pointers should be cast to the correct type before calling them, rather
than calling them with the wrong type and trying to cast the result. Fixes a warning compiling
mail.c on 64 bit architectures, and this might well have caused a crash when using the qmail plugin
on those architectures too.
