MFC: r290549,r290729

r290549:

Reorganize code to elimitate one _sseek() call for append modes.

r290729:

1. Remove my overcomplicated error fallback and just return error

immediatelly as old code does, now for append modes too.
Real use case for such fallback is impossible (unless specially crafted).

2. Remove now unneded include I forgot to remove in prev. commits.