New "m" file open mode: non-truncating "w".

Quite surprisingly ISO C lacks a way in fopen to open a file
for writing such that it is not truncated if it already
exists, and not opened in append mode.

(But you will be glad to know that ISO C is adding incredibly
useful features in this area, like Microsoft's fopen_s!)

Let us add modes "m" and "m+" which will be like "w" and "w+",
but without the truncation to zero length (no O_TRUNC
is passed to open).

* stream.c (w_fopen_mode): New static function.
(open_file, open_tail, tail_strategy): Use w_fopen_mode
instead of directly calling w_fopen.
(do_parse_mode): Handle 'm' and set new notrunc flag.

* stream.h (struct stdio_mode): New member, notrunc flag.
(stdio_mode_init_blank, stdio_mode_init_r,
stdio_mode_init_rpb): Initializer macros updated to include
initializer for notrunc flag.

* txr.1: Documented "m" mode.

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