maktaba#syscall doesn't support newlines properly

[dbarnett]

The makataba#syscall functions seem to garble newlines you try to pass to them. For example:

:echo maktaba#syscall#Create(['echo', "foo\nbar"]).GetCommand()

echo 'foo\
bar'

I think it will work properly if we just changed it to pass newlines through unmodified inside single-quoted strings.