indent(1): don't treat bare '_t' as a type name when -ta is used

It seems that identifier "_t" is sometimes used as a variable name,
even in our tree. Not that I endorse that, but still it's better
to require at least one character before _t suffix to consider
an identifier to be a type name.

Reported by: Alex Vasylenko <lxv@omut.org>
MFC after: 1 week