gdb: make the target_sections table private within program_space

Following on from earlier commits which made access to the
target_sections table more 'const', this commit makes the table
private within the program_space class and provides member functions
to access the table.

Ideally I would have liked for the new target_sections member
function (on program_space) to return a 'const' reference to the table
within the program_space.  Unfortunately, there are two places in
solib-*.c, where code outside of the program_space class modifies the
target_sections table, and so to support this we need to return a
non-const reference.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* exec.c (exec_target::close): Call new clear_target_sections
	function.
	(program_space::add_target_sections): Update name of member
	variable.
	(program_space::foreach_target_section): New function.
	(program_space::add_target_sections): Update name of member
	variable.
	(program_space::remove_target_sections): Likewise.
	(exec_one_fork): Use new target_sections member function.
	(exec_target::get_section_table): Likewise.
	(exec_target::files_info): Likewise.
	(set_section_command): Use new foreach_target_section member
	function.
	(exec_set_section_address): Likewise.
	(exec_target::has_memory): Use new target_sections member
	function.
	* progspace.h (program_space::clear_target_sections): New member
	function.
	(program_space::target_sections): Rename member variable to
	m_target_sections, replace with a new member function.
	(program_space::foreach_target_section): Declare new member
	function.
	(program_space::m_target_sections): New member variable.
	* solib-dsbt.c (scan_dyntag): Use new member function.
	* solib-svr4.c (scan_dyntag): Likewise.

gdb/solib-dsbt.c

@@ -425,7 +425,7 @@ scan_dyntag (int dyntag, bfd *abfd, CORE_ADDR *ptr)
 
   bool found = false;
   for (const target_section &target_section
-	 : current_program_space->target_sections)
+	 : current_program_space->target_sections ())
     if (sect == target_section.the_bfd_section)
       {
 	dyn_addr = target_section.addr;