Fill paragraphs within the region, allowing varying indentation within each.
This command divides the region into "paragraphs",
only at paragraph-separator lines, then fills each paragraph
using as the fill prefix the smallest indentation of any line
in the paragraph.
When calling from a program, pass range to fill as first two arguments.
Optional third and fourth arguments JUSTIFYP and CITATION-REGEXP:
JUSTIFYP to justify paragraphs (prefix arg).
When filling a mail message, pass a regexp for CITATION-REGEXP
which will match the prefix of a line which is a citation marker
plus whitespace, but no other kind of prefix.
Also, if CITATION-REGEXP is non-nil, don't fill header lines.