[gdb] Print user/includes fields for maint commands

The type struct compunit_symtab contains two fields (disregarding field next)
that express relations with other compunit_symtabs: user and includes.

These fields are currently not printed with "maint info symtabs" and
"maint print symbols".

Fix this such that for "maint info symtabs" we print:
...
   { ((struct compunit_symtab *) 0x23e8450)
     debugformat DWARF 2
     producer (null)
     dirname (null)
     blockvector ((struct blockvector *) 0x23e8590)
+    user ((struct compunit_symtab *) 0x2336280)
+    ( includes
+      ((struct compunit_symtab *) 0x23e85e0)
+      ((struct compunit_symtab *) 0x23e8960)
+    )
         { symtab <unknown> ((struct symtab *) 0x23e85b0)
           fullname (null)
           linetable ((struct linetable *) 0x0)
         }
   }
...

And for "maint print symbols" we print:
...
-Symtab for file <unknown>
+Symtab for file <unknown> at 0x23e85b0
 Read from object file /data/gdb_versions/devel/a.out (0x233ccf0)
 Language: c

 Blockvector:

 block #000, object at 0x23e8530, 0 syms/buckets in 0x0..0x0
   block #001, object at 0x23e84d0 under 0x23e8530, 0 syms/buckets in 0x0..0x0

+Compunit user: 0x2336300
+Compunit include: 0x23e8900
+Compunit include: 0x23dd970
...
Note: for user and includes we don't list the actual compunit_symtab address,
but instead the corresponding symtab address, which allows us to find that
symtab elsewhere in the output (given that we also now print the address of
symtabs).

gdb/ChangeLog:

2020-03-25  Tom de Vries  <tdevries@suse.de>

	* symtab.h (is_main_symtab_of_compunit_symtab): New function.
	* symmisc.c (dump_symtab_1): Print user and includes fields.
	(maintenance_info_symtabs): Same.