[SV 65268] Un-set append mode for stdout/stderr on exit

[SV 65268] Un-set append mode for stdout/stderr on exit

It turns out that options set on stdout/stderr last after exit.
Leaving append-mode enabled can break other facilities, so reset the
flags on stdout/stderr before we exit.

* src/os.h: Add a new fd_reset_append() to reset flags on FDs.
Modify fd_set_append() to return the old flags.
* src/posixos.c (fd_reset_append): Set provided flags on the FD.
(fd_set_append): Return the previous flags set on the FD.
* src/output.c (output_init): Preserve old flags for stdout/stderr.
(output_close): Reset flags for stdout/stderr.
* src/w32/w32os.c: Implement dummy methods.