Fix "is a record target open" checks.

Fix "is a record target open" checks.

RECORD_IS_USED and record_full_open look at current_target.to_stratum
to determine whether a record target is in use.  This is wrong because
arch_stratum is greater than record_stratum, so if an arch_stratum
target is pushed, RECORD_IS_USED and record_full_open will miss it.

To fix this, we can use the existing find_record_target instead, which
looks up for a record stratum target across the target stack.  Since
that means exporting find_record_target in record.h, RECORD_IS_USED
ends up redundant, so the patch eliminates it.

That exercise then reveals other issues:

- adjust_pc_after_break is gating record_full_... calls based on
RECORD_IS_USED.  But, record_full_ calls shouldn't be made when
recording with the record-btrace target.  So this adds a new
record_full_is_used predicate to be used in that spot.

- record_full_open says "Process record target already running", even
if the recording target is record-btrace ("process record" is the
original complete name of the record-full target).  record_btrace_open
only says "The process is already being recorded." and does not
suggest "record stop", like record-full does.  The patch factors out
and merges that error to a new record_preopen function that all record
targets call in their open routine.

Tested on x86_64 Fedora 17.

gdb/
2014-01-14  Pedro Alves  <palves@redhat.com>
	    Tom Tromey  <tromey@redhat.com>

	* infrun.c (use_displaced_stepping): Use find_record_target
	instead of RECORD_IS_USED.
	(adjust_pc_after_break): Use record_full_is_used instead of
	RECORD_IS_USED.
	* record-btrace.c (record_btrace_open): Call record_preopen
	instead of checking RECORD_IS_USED.
	* record-full.c (record_full_shortname)
	(record_full_core_shortname): New globals.
	(record_full_is_used): New function.
	(find_full_open): Call record_preopen instead of checking
	RECORD_IS_USED.
	(init_record_full_ops): Set the target's shortname to
	record_full_shortname.
	(init_record_full_core_ops): Set the target's shortname to
	record_full_core_shortname.
	* record-full.h (record_full_is_used): Declare.
	* record.c (find_record_target): Make extern.
	(record_preopen): New function.
	* record.h (RECORD_IS_USED): Delete macro.
	(find_record_target, record_preopen): Declare functions.