btrace: Resume recording after disconnect.

btrace: Resume recording after disconnect.

This patch allows gdbserver to continue recording after disconnect.  On
reconnect, the recorded data is accessible to gdb as if no disconnect happened.

A possible application for this feature is remotely examine bugs that occur
at irregular intervals, where maintaining a gdb connection is inconvenient.

This also fixes the issue mentioned here:
https://sourceware.org/ml/gdb-patches/2015-11/msg00424.html

Signed-off-by: Tim Wiederhake <tim.wiederhake@intel.com>
gdb/ChangeLog:
	* NEWS: Resume btrace on reconnect.
	* record-btrace.c: Added record-btrace.h include.
	(record_btrace_open): Split into this and ...
	(record_btrace_push_target): ... this.
	(record_btrace_disconnect): New function.
	(init_record_btrace_ops): Use record_btrace_disconnect.
	* record-btrace.h: New file.
	* remote.c: Added record-btrace.h include.
	(remote_start_remote): Check recording status.
	(remote_btrace_maybe_reopen): New function.

gdb/doc/ChangeLog:
	* gdb.texinfo: Resume btrace on reconnect.

gdb/testsuite/ChangeLog:

	* gdb.btrace/reconnect.c: New file.
	* gdb.btrace/reconnect.exp: New file.

Change-Id: I95e8b0ab8a89e58591aba0e63818cee82fd211bc