Convert probes to type-safe registry API

Convert probes to type-safe registry API

This changes the probes code in elfread.c to use the type-safe
registry API.  While doing this, I saw that the caller of get_probes
owns the probes, so I went through the code and changed the vectors to
store unique_ptrs, making the ownership relationship more clear.

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

	* symfile.h (struct sym_probe_fns) <sym_get_probes>: Change type.
	* symfile-debug.c (debug_sym_get_probes): Change type.
	* stap-probe.c (handle_stap_probe):
	(stap_static_probe_ops::get_probes): Change type.
	* probe.h (class static_probe_ops) <get_probes>: Change type.
	* probe.c (class any_static_probe_ops) <get_probes>: Change type.
	(parse_probes_in_pspace): Update.
	(find_probes_in_objfile, find_probe_by_pc, collect_probes):
	Update.
	(any_static_probe_ops::get_probes): Change type.
	* elfread.c (elfread_data): New typedef.
	(probe_key): Change type.
	(elf_get_probes): Likewise.  Update.
	(probe_key_free): Remove.
	(_initialize_elfread): Update.
	* dtrace-probe.c (class dtrace_static_probe_ops) <get_probes>:
	Change type.
	(dtrace_process_dof_probe, dtrace_process_dof)
	(dtrace_static_probe_ops::get_probe): Change type.