Test case to detect recursive unwinding in Python-based unwinders.

This test case verifies that GDB will not attempt to invoke a python
unwinder recursively.

At the moment, the behavior exhibited by GDB looks like this:

    (gdb) source py-recurse-unwind.py
    Python script imported
    (gdb) b ccc
    Breakpoint 1 at 0x4004bd: file py-recurse-unwind.c, line 23.
    (gdb) run
    Starting program: py-recurse-unwind
    TestUnwinder: Recursion detected - returning early.
    TestUnwinder: Recursion detected - returning early.
    TestUnwinder: Recursion detected - returning early.
    TestUnwinder: Recursion detected - returning early.

    Breakpoint 1, ccc (arg=<unavailable>) at py-recurse-unwind.c:23
    23      }
    (gdb) bt
    #-1 ccc (arg=<unavailable>) at py-recurse-unwind.c:23
    Backtrace stopped: previous frame identical to this frame (corrupt stack?)

[I've shortened pathnames for easier reading.]

The desired / expected behavior looks like this:

    (gdb) source py-recurse-unwind.py
    Python script imported
    (gdb) b ccc
    Breakpoint 1 at 0x4004bd: file py-recurse-unwind.c, line 23.
    (gdb) run
    Starting program: py-recurse-unwind

    Breakpoint 1, ccc (arg=789) at py-recurse-unwind.c:23
    23      }
    (gdb) bt
    #0  ccc (arg=789) at py-recurse-unwind.c:23
    #1  0x00000000004004d5 in bbb (arg=456) at py-recurse-unwind.c:28
    #2  0x00000000004004ed in aaa (arg=123) at py-recurse-unwind.c:34
    #3  0x00000000004004fe in main () at py-recurse-unwind.c:40

Note that GDB's problems go well beyond the fact that it invokes the
unwinder recursively.  In the process it messes up some internal state
(the frame stash) leading to display of (only) the sentinel frame in
the backtrace.

gdb/testsuite/ChangeLog:

	* gdb.python/py-recurse-unwind.c: New file.
	* gdb.python/py-recurse-unwind.py: New file.
	* gdb.python/py-recurse-unwind.exp: New file.