greatly improve freopen behavior

1. don't open /dev/null just as a basis to copy flags; use shared
__fmodeflags function to get the right file flags for the mode.

2. handle the case (probably invalid, but whatever) case where the
original stream's file descriptor was closed; previously, the logic
re-closed it.

3. accept the "e" mode flag for close-on-exec; update dup3 to fallback
to using dup2 so we can simply call __dup3 instead of putting fallback
logic in freopen itself.

src/internal/stdio_impl.h

@@ -87,6 +87,7 @@ size_t __fwritex(const unsigned char *, size_t, FILE *);
 int __putc_unlocked(int, FILE *);
 
 FILE *__fdopen(int, const char *);
+int __fmodeflags(const char *);
 
 #define OFLLOCK() LOCK(libc.ofl_lock)
 #define OFLUNLOCK() UNLOCK(libc.ofl_lock)