Retrospective commit from 2009-09-20.

Retrospective commit from 2009-09-20.

Handle positions at BEGV and ZV.  Mor initialization cleanups.

 xdisp.c (reseat_1): Handle position < BEGV.
 bidi.c (bidi_paragraph_init): Set bidi_it->ch_len.  Handle ZV.
 (bidi_init_it): Don't initialize bidi_it->ch_len.
 (bidi_resolve_explicit_1): Abort if bidi_it->ch_len was not
 initialized.
 (bidi_at_paragraph_end, bidi_resolve_explicit_1)
 (bidi_resolve_weak, bidi_level_of_next_char): Handle bytepos at
 ZV_BYTE.
 (bidi_resolve_explicit_1): Handle position < BEGV.