Flip user_string_n_quoted to limit the input count

Flip user_string_n_quoted to limit the input count

The documentation of that function implies that it's counting n from the
input string, but in fact that limit was being applied to the output,
including quotes and escaping.

Now _stp_text_str takes two length parameters to limit the input count
and output size separately.  A new user_string_n2_quoted() lets you
specify both of these lengths, and user_string_n_quoted now uses that to
limit input length, or output still in compatibility mode.

Several syscall tapsets which read user buffers of known length will now
use user_string_n2_quoted to fit that.  This was seen in syscall.write
which may not necessarily have any '\0' at the end of its buffer.