Fix bug #17892 with mode/header line and display margins.

Fix bug #17892 with mode/header line and display margins.

 src/dispnew.c (prepare_desired_row): Accept 2 additional arguments:
 the window whose glyph row is being prepared and a flag whether it
 is for mode/header line.  Make sure the glyph row's marginal areas
 are in sync with what the window wants.
 src/xdisp.c (display_line, display_mode_line): Call
 prepare_desired_row with additional arguments, as appropriate.
 src/dispextern.h (prepare_desired_row): Adjust prototype.
 src/window.h: Improve commentary of the marginal columns.