pr: ensure the page header line is of the required format

pr: ensure the page header line is of the required format

Before this change, with too long a file name, the name would
abut the date field on the left and possibly also the "Page N"
field on the right, rather than leaving a one-space separator
in each case.  Fixes a regression introduced on Mar 6 2009,
by commit a4053c5291d5797734b3e4f042f9e1adf3944fd6

* src/pr.c (print_header): Ensure that there is at least one
space before and after the file name part of the header line.
* NEWS: Mention it.
* tests/pr/W20l24f-ll: s/xPage/ x Page/.
* THANKS: Update.
Reported by Denis McKeon, in https://savannah.gnu.org/bugs/?28492.