Problem:
When `:!` writes shell output to a buffer, write_output() splits on `\r`, `\n`,
and `\r\n`, replacing the terminator byte with NUL. For a binary-mode buffer
this is wrong: `\r` should be preserved verbatim, not treated as a line
terminator. This wrong behavior causes a file like `\r\n` round-trips through
`:%!cat` to `\n`.
This was masked when 'shelltemp' was enabled, because output went through a temp
file and the regular file I/O path handled binary-mode correctly. Switching the
default to 'noshelltemp' exposed the bug, since output is now piped directly
into write_output().
Solution:
In `write_output()`, skip the `\r` and `\r\n` splits for a binary-mode buffer;
only split on `\n`.
832a68835b