Add -V ktpipe: ktrace-friendly pipe/socketpair.

When -V ktpipe is in effect, pipe() and socketpair() show up in ktrace
output in a way that indicates the file descriptors they return: such a
call does a stat() on a pathname of the form "/\\/%s" where the string
is generated by a printf-style format-and-args passed by the caller;
this gets the string into kdump output via a NAMI line.  Then it calls
close() with first the [0] descriptor, then the [1] descriptor, each
|ed with 0x40000000 (so the close() fails harmlessly, but the argument
in kdump output gives the fd numbers).

When -V ktpipe is not in effect, none of this happens.