Remove ALL_SO_LIBS and so_list_head

Remove ALL_SO_LIBS and so_list_head

This patch started as an attempt to replace ALL_SO_LIBS with an
ordinary C++ iterator.  However, then I tripped over the so_list_head
define again, and decided to remove it as well.

gdb/ChangeLog
2020-05-08  Tom Tromey  <tom@tromey.com>

	* mi/mi-cmd-file.c (mi_cmd_file_list_shared_libraries): Update.
	* solib-svr4.c (svr4_fetch_objfile_link_map): Update.
	(enable_break): Update.
	* solib-frv.c (frv_fdpic_find_global_pointer): Update.
	(frv_fdpic_find_canonical_descriptor): Update.
	(frv_fetch_objfile_link_map): Update.
	* progspace.c (program_space::free_all_objfiles): Update.
	(program_space::solibs): New method.
	* progspace.h (struct program_space) <solibs>: New method.
	* solist.h (master_so_list): Don't declare.
	(ALL_SO_LIBS): Remove.
	* solib.h (so_list_head): Remove.
	(update_solib_list): Update comment.
	* solib.c (master_so_list): Remove.
	(solib_used, update_solib_list, solib_add)
	(info_sharedlibrary_command, clear_solib)
	(reload_shared_libraries_1, remove_user_added_objfile): Update.