Fix bug #7038 with cursor motion in paragraphs w/o strong characters.

Fix bug #7038 with cursor motion in paragraphs w/o strong characters.

 xdisp.c (Fcurrent_bidi_paragraph_direction): Call
 bidi_paragraph_init with NO_DEFAULT_P non-zero.
 bidi.c (bidi_paragraph_init): Accept an additional argument
 NO_DEFAULT_P; all callers changed.  If NO_DEFAULT_P is non-zero,
 search back until a paragraph with a strong directional character
 is found, and use that to determine paragraph's base direction.
 dispextern.h (bidi_paragraph_init): Update prototype.