Fix buffer overrun problem reported by TAKAI Kousuke, along

Fix buffer overrun problem reported by TAKAI Kousuke, along
with some other POSIX incompatibilities.

(print_esc): Do not treat \x specially if
POSIXLY_CORRECT.  Avoid buffer overrun if the format ends
in backslash.  Treat incomplete escape sequences as strings
of characters, as POSIX requires.
(print_formatted): Allow multiple flags.  Avoid buffer overrun
if the format is incomplete.