pr: fix a bug whereby --indent=N (-o) did not indent header lines

pr: fix a bug whereby --indent=N (-o) did not indent header lines

* src/pr.c (print_header): Honor chars_per_margin also for the header.
Inspired by a patch from C de-Avillez.
This change also adjusts two other %*s directives to be %*.*s instead,
(for the padding on either side of "file_text") to correct a bug that
could make pr print one space of padding instead of none.
* NEWS (Bug fixes): Mention it.
* tests/pr/o3Jml24f-lm-lo: Adjust for new expected output.
* tests/pr/o3a3Sl24f-tn: Likewise.
* tests/pr/o3a3Snl24f-tn: Likewise.
* tests/pr/o3a3l24f-tn: Likewise.
* tests/pr/o3b3Sl24f-tn: Likewise.
* tests/pr/o3b3Snl24f-tn: Likewise.
* tests/pr/o3b3l24f-tn: Likewise.
* tests/pr/o3mSl24f-bl-tn: Likewise.
* tests/pr/o3mSnl24fbltn: Likewise.
* tests/pr/o3ml24f-bl-tn: Likewise.