Indent each nonblank line in the region.
A numeric prefix argument specifies a column: indent each line to that column.
With no prefix argument, the command chooses one of these methods and
indents all the lines with it:
1) If `fill-prefix' is non-nil, insert `fill-prefix' at the
beginning of each line in the region that does not already begin
with it.
2) If `indent-region-function' is non-nil, call that function
to indent the region.
3) Indent each line via `indent-according-to-mode'.
Called from a program, START and END specify the region to indent.
If the third argument COLUMN is an integer, it specifies the
column to indent to; if it is nil, use one of the three methods above.