Fill each of the paragraphs in the region.
A prefix arg means justify as well.
The `fill-column' variable controls the width.
Noninteractively, the third argument JUSTIFY specifies which
kind of justification to do: `full', `left', `right', `center',
or `none' (equivalent to nil). A value of t means handle each
paragraph as specified by its text properties.
The fourth arg NOSQUEEZE non-nil means to leave whitespace other
than line breaks untouched, and fifth arg TO-EOP non-nil means
to keep filling to the end of the paragraph (or next hard newline,
if variable `use-hard-newlines' is on).
Return the fill-prefix used for filling the last paragraph.
If `sentence-end-double-space' is non-nil, then period followed by one
space does not end a sentence, so don't break a line there.