vimcat: use fifo not file, even on cygwin etc.

In 09fd981 the tail -f call for reading the fifo was replaced with a cat
loop, this works fine on cygwin and presumably everywhere else it didn't
work before, so remove the OS check for making a regular file instead of
a fifo and always make a fifo instead.

This also fixes vimcat on cygwin/msys since 09fd981 broke it, the cat
loop would repeat the file over and over.

vimcat

@@ -179,15 +179,7 @@ trap 'quit 1' PIPE HUP INT QUIT ILL TRAP KILL BUS TERM
 tmp_file_in=$tmp_dir/vimcat_in.txt
 out_fifo=$tmp_dir/vimcat_out.fifo
 
-case $(uname -s) in
-    SunOS*|CYGWIN*|MINGW*|MSYS*)
-        # the fifo streaming doesn't work on windows and solaris
-        touch "$out_fifo"
-        ;;
-    *)
-        mkfifo "$out_fifo"
-        ;;
-esac
+mkfifo "$out_fifo"
 
 # check for arguments
 while [ $# -gt 0 ] ; do