better \t in uniform mode ; signal sentence end

Sentence endings will be important in K-P because
we want to do some discouraging and encouraging of
linebreaks based on them.

In uniform mode, instead of rewriting tabs, we detect
them and indicate sentence end, which causes two spaces
to be emitted.

Did some performance comparisons and we're hanging at
~20% slower than GNU fmt. However, GNU fmt chokes
*big time* on huge input files when in -c or -t mode,
whereas we are slowest in -u mode.

TODO: benchmark against OpenBSD and NetBSD fmt