sim: ppc: track closed state of file descriptors 0, 1, and 2.

sim: ppc: track closed state of file descriptors 0, 1, and 2.

This change tracks the "closed" state of file descriptors 0, 1, and 2,
introducing the function fdbad() to emul_netbsd.c and emul_unix.c.
Note that a function of the same name and purpose exists in
sim/common/callback.c.

This patch eliminates all of the "unresolved testcases" when testing
GDB against the powerpc simulator.

This occurs because the powerpc simulator closes, on behalf of the
testcase, the file descriptors associated with stdin, stdout, and
stderr.  GDB still needs these descriptors to communicate with the
user or, in this case, with the testing framework.