[merb-core] Forking class reloading fix for Ruby 1.9.x.
[merb-core] Forking class reloading fix for Ruby 1.9.x.

On Ruby 1.9.x (tested on 1.9.1 and 1.9.2dev) class reloading failded to
work. After every single save it just hanged after creating new child.

This patch fixes the problem with properly closing the pipes and using
Process.waitpid instead of Proces.detatch.

Patch also rise rate at Kernel.select checks the output from the child
which lowers system requirements for the class reloading.

merb/merb-core,merb/merb-core