btrace: control memory access during replay

The btrace record target does not trace data.  We therefore do not allow
accessing read-write memory during replay.

In some cases, this might be useful to advanced users, though, who we assume
to know what they are doing.

Add a set|show command pair to turn this memory access restriction off.

	* record-btrace.c (record_btrace_allow_memory_access): Remove.
	(replay_memory_access_read_only, replay_memory_access_read_write)
	(replay_memory_access_types, replay_memory_access)
	(set_record_btrace_cmdlist, show_record_btrace_cmdlist)
	(cmd_set_record_btrace, cmd_show_record_btrace)
	(cmd_show_replay_memory_access): New.
	(record_btrace_xfer_partial, record_btrace_insert_breakpoint)
	(record_btrace_remove_breakpoint): Replace
	record_btrace_allow_memory_access with replay_memory_access.
	(_initialize_record_btrace): Add commands.
	* NEWS: Announce it.

testsuite/
	* gdb.btrace/data.exp: Test it.

doc/
	* gdb.texinfo (Process Record and Replay): Document it.
test, gcore: move capture_command_output into lib/gdb.exp

Allow gcore's capture_command_output function to be used by other tests.

testsuite/
	* gdb.base/gcore.exp (capture_command_output): Move ...
	* lib/gdb.exp (capture_command_output): ... here.
btrace, vdso: add vdso target sections

When loading symbols for the vdso, also add its sections to target_sections.

This fixes an issue with record btrace where vdso instructions could not be
disassembled during replay.

	* symfile-mem.c (symbol_file_add_from_memory): Add BFD sections.

testsuite/
	* gdb.btrace/vdso.c: New.
	* gdb.btrace/vdso.exp: New.