Bug fix for 4.2BSD (and earlier?) make

[Greg Noel]

A bug has been found in the 4.2 version of make.  The symptoms are
various kinds of flakeyness in large makefiles or in makefiles that
reference a lot of different directories for files.  The problem
is that after the vfork to execute the command the child would mark
the parent's files as closed.  If make needed to look in the directory
again, it would reopen it.  Eventually, make (or a child) would run
out of avalilable file descriptors.  The cure is to replace subroutine
doclose in the file dosys.c with the following code:
--------------------------- cut here ------------------------------------
doclose()	/* Close open directory files before exec'ing */
{
register struct dirhdr *od;
/*		Original code:
for (od = firstod; od; od = od->nxtopendir)
	if (od->dirfc != NULL) {
		closedir(od->dirfc);
		od->dirfc = NULL;
	}
	We have done a vfork, so we can't do anything to stomp on our
	parent's address space.  Unfortunately, the code above doesn't
	work since our parent's files remain open.  And we can't just
	do a closedir, since it will free a dynamicly-allocated table
	entry.  So we use this hack, calling close directly.  If the
	interface ever changes, we are going to be out of luck.
*/
for (od = firstod; od; od = od->nxtopendir)
	if (od->dirfc != NULL)
		close(od->dirfc->dd_fd);
}