Refactor fork()-related code. We need to do various housekeeping tasks

before we can invoke fork() -- flush stdio buffers, save and restore the
profiling timer on Linux with LINUX_PROFILE, and handle BeOS stuff. This
patch moves that code into a single function, fork_process(), instead of
duplicating it at the various callsites of fork().

This patch doesn't address the EXEC_BACKEND case; there is room for
further cleanup there.