[boost] Update [Was: Boost.Process 0.5 released]

I've updated code and documentation at <http://www.highscore.de/boost/process0.5/>: - On POSIX set_on_error and throw_on_error report now if execve() failed in the forked process. - return_on_error has been removed. - set_env has been added to make it possible to set environment variables. - close_fds_except has been changed to close_fds_if (it expects now a predicate). - The executors call now also an initializer function after a system call returned successfully. More changes to come. Boris