Make core files the process_stratum.
* corefile.c (core_target): New variable.
(core_file_command): Remove variable t, use core_target.
* corelow.c (core_ops): Make it static.
(init_core_ops): Change to process_stratum. Initialize CORE_TARGET.
* defs.h (make_cleanup_unpush_target): New prototype.
* gdbarch.h: Regenerate.
* gdbarch.sh (core_pid_to_str): Remove core_stratum from its comment.
* gdbcore.h (core_target): New declaration.
* inf-ptrace.c (inf_ptrace_create_inferior, inf_ptrace_attach): New
variables ops_already_pushed and back_to. Use push_target,
make_cleanup_unpush_target and discard_cleanups calls.
* record.c (record_open): Replace core_stratum by a core_bfd check.
* target.c (target_is_pushed): New function.
(find_core_target): Remove.
* target.h (enum strata) <core_stratum>: Remove.
(target_is_pushed): New declaration.
(find_core_target): Remove declaration.
* tracepoint.c (init_tfile_ops) <to_stratum>: Remove comment.
* utils.c (do_unpush_target, make_cleanup_unpush_target): New functions.
gdb/doc/
Make core files the process_stratum.
* gdb.texinfo (Active Targets): Remove core_stratum. Include
record_stratum example.
gdb/testsuite/
Make core files the process_stratum.
* gdb.base/corefile.exp (run: load core again)
(run: sanity check we see the core file, run: with core)
(run: core file is cleared, attach: load core again)
(attach: sanity check we see the core file, attach: with core)
(attach: core file is cleared): New tests.
* gdb.base/coremaker.c (main): New parameters. Implement "sleep" argv.
