Introduce gdb_dlhandle_up

Introduce gdb_dlhandle_up

This introduces gdb_dlhandle_up, a unique_ptr that can close a
dlopen'd library.  All the functions working with dlopen handles are
updated to use this new type.

I did not try to build this on Windows.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* jit.c (struct jit_reader): Declare separately.  Add constructor
	and destructor.  Change type of "handle".
	(loaded_jit_reader): Define separately.
	(jit_reader_load): Update.  New "new".
	(jit_reader_unload_command): Use "delete".
	* gdb-dlfcn.h (struct dlclose_deleter): New.
	(gdb_dlhandle_up): New typedef.
	(gdb_dlopen, gdb_dlsym): Update types.
	(gdb_dlclose): Remove.
	* gdb-dlfcn.c (gdb_dlopen): Return a gdb_dlhandle_up.
	(gdb_dlsym): Change type of "handle".
	(make_cleanup_dlclose): Remove.
	(dlclose_deleter::operator()): Rename from gdb_dlclose.
	* compile/compile-c-support.c (load_libcc): Update.