Add 'temporary file' facility to fd.c, and arrange for temp

Add 'temporary file' facility to fd.c, and arrange for temp
files to be closed automatically at transaction abort or commit, should
they still be open.  Also close any still-open stdio files allocated with
AllocateFile at abort/commit.  This should eliminate problems with leakage
of file descriptors after an error.  Also, put in some primitive buffered-IO
support so that psort.c can use virtual files without severe performance
penalties.