pr -F no longer suppresses the footer or the first two blank header lines

pr -F no longer suppresses the footer or the first two blank header lines

Here's a patch along the lines of
<http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/10849/focus=10857>.
It's a bit large since it changes the test cases to match the behavior,
and the test cases' file names depend on the behavior!

* NEWS: pr -F no longer suppresses the footer or the first two blank
lines in the header.
* doc/coreutils.texi (pr invocation): Likewise.  Also, a too-short
page length implies -t, not -T.
* src/pr.c (lines_per_header, lines_per_footer): Now constants.
(init_parameters): Don't try to change them.
(print_header): Use the same header and footer format regardless of
wither form feeds are being used.
(usage): Adjust to above change when describing too-short page length.
Too-short page length impliesy -t, not -T.
* tests/pr/2-S_f-t_notab: Adjust to the fact that -F now affects
only formfeed handling; it does not change the header.
* tests/pr/2-Sf-t_notab: Likewise.
...
* tests/pr/3-5l17f-t: Remove, since it's been renamed to another
file whose name has a line count 7 larger,
reflecting the new line count needed for this behavior.
* tests/pr/3a3l8f-t: Likewise.
...
* tests/pr/w72l24f-ll: Likewise.

Signed-off-by: Jim Meyering <jim@meyering.net>