section_table_xfer_memory: Replace section name with callback predicate

This patch is motivated by the need to be able to select sections
that section_table_xfer_memory_partial should consider for memory
transfers.  I'll use this facility in the next patch in this series.

section_table_xfer_memory_partial() can currently be passed a section
name which may be used to make name-based selections.  This is similar
to what I want to do, except that I want to be able to consider
section flags instead of the name.

I'm replacing the section name parameter with a predicate that,
when passed a pointer to a target_section struct, will return
true if that section should be further considered, or false which
indicates that it shouldn't.

I've converted the one existing use where a non-NULL section
name is passed to section_table_xfer_memory_partial().   Instead
of passing the section name, it now looks like this:

	  auto match_cb = [=] (const struct target_section *s)
	    {
	      return (strcmp (section_name, s->the_bfd_section->name) == 0);
	    };

	  return section_table_xfer_memory_partial (readbuf, writebuf,
						    memaddr, len, xfered_len,
						    table->sections,
						    table->sections_end,
						    match_cb);

The other callers all passed NULL; they've been simplified somewhat
in that they no longer need to pass NULL.

gdb/ChangeLog:

	* exec.h (section_table_xfer_memory): Revise declaration,
	replacing section name parameter with an optional callback
	predicate.
	* exec.c (section_table_xfer_memory): Likewise.
	* bfd-target.c, exec.c, target.c, corelow.c: Adjust all callers
	of section_table_xfer_memory.

gdb/bfd-target.c

@@ -77,8 +77,7 @@ target_bfd::xfer_partial (target_object object,
 	return section_table_xfer_memory_partial (readbuf, writebuf,
 						  offset, len, xfered_len,
 						  m_table.sections,
-						  m_table.sections_end,
-						  NULL);
+						  m_table.sections_end);
       }
     default:
       return TARGET_XFER_E_IO;