Convert solib-svr4.c to type-safe registry API

Convert solib-svr4.c to type-safe registry API

This changes solib-svr4y.c to use the type-safe registry API.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* solib-svr4.c (struct svr4_info): Add initializers and
	destructor.
	<probes_table>: Now an htab_up.
	(solib_svr4_pspace_data): Change type.
	(free_probes_table): Simplify.
	(~svr4_info): Rename from svr4_pspace_data_cleanup.
	(get_svr4_info, probes_table_htab_remove_objfile_probes)
	(probes_table_remove_objfile_probes, register_solib_event_probe)
	(solib_event_probe_at, svr4_update_solib_event_breakpoint)
	(_initialize_svr4_solib): Update.