linux-nat: introduce pending_status_str

I noticed that some debug log output printing an lwp's pending status
wasn't considering lp->waitstatus.  This fixes it, by introducing a
new pending_status_str function.

Also fix the comment in gdb/linux-nat.h describing
lwp_info::waitstatus and details the description of lwp_info::status
while at it.

Change-Id: I66e5c7a363d30a925b093b195d72925ce5b6b980
Approved-By: Andrew Burgess <aburgess@redhat.com>