Add StopRequestPending field/accessors to Thread.
Fix #9854.

The post syscall debug events used for output capture have an
unfortunate side effect: when asked to debug a thread, the thread is
interrupted, which, if currently blocked in a syscall will cause it to
unblock and send a post syscall event indicating such. However, this
will also absorb the debug stop flag that was set by the initial debug
request, and so we won't actually get the separate event indicating
thread debugged.

Consequently, we now set a pending stop request flag on the
corresponding Thread object, and check if it's set when processing
syscall events. If so, we treat such an event as having triggered a
debug stop even though the received event type is not explicitly
B_DEBUGGER_MESSAGE_THREAD_DEBUGGED.
Don't try to read data for output capture if the syscall didn't succeed.
Call debug_cleanup() before converting kernel_args to 64-bit addresses, fixes #9842.

In debug_cleanup(), if the debug syslog buffer is disabled (the default when
KDEBUG_LEVEL is 0), then a new buffer is allocated with kernel_args_malloc().
This is done after kernel_args addresses have been converted to 64-bit, so
the address the kernel gets will be 32-bit, resulting in the page fault seen
in #9842. Fixed by moving the call to debug_cleanup() to before
convert_kernel_args().