uip/scansbr.c: Stop `scan -file' copying mailbox to /dev/null.

uip/scansbr.c: Stop `scan -file' copying mailbox to /dev/null.

scan() was copying the mailbox to /dev/null when called for `scan -file
foo.mbox' because it used outnum being non-zero as the test of whether
to write instead of FILE pointer scnout being non-NULL.  By switching,
we can avoid the opening of /dev/null that was just to give scnout a
valid value for the needless fputs(3), etc.

Add comment explaining scan()'s outnum input parameter's three
functions: -1, 0, and positive.