fix recursively_compute_inclusions and add dwz test case

fix recursively_compute_inclusions and add dwz test case

Doug pointed out a while ago that in the final dwz -m patch, nothing
ever set symtab::user.

This patch fixes this oversight and adds a test case showing why it is
important.

Built and regtested (both ways) on x86-64 Fedora 18.
The new test unconditionally tests the partial unit machinery, which I
think is an added plus.

	* dwarf2read.c (recursively_compute_inclusions): Add
	"immediate_parent" argument.  Set symtab's "user" field
	if not set.
	(compute_symtab_includes): Update.

	* gdb.dwarf2/dwz.exp: New file.