pgsql: modify segno. for pg_walfile_name() and pg_walfile_name_offset()

modify segno. for pg_walfile_name() and pg_walfile_name_offset()

Previously these functions returned the previous segment number if the
LSN was on a segment boundary.  We now always return the current segment
number for an LSN.

Docs updated to reflect this change.  Regression tests added, author
Andres Freund.

Also mentioned in thread
https://postgr.es/m/flat/20220204225057.GA1535307%40nathanxps13#d964275c9540d8395e138efc0a75f7e8

BACKWARD INCOMPATIBILITY

Reported-by: Kyotaro Horiguchi

Discussion: https://postgr.es/m/20190726.172120.101752680.horikyota.ntt@gmail.com

Co-authored-by: Kyotaro Horiguchi

Backpatch-through: master

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/344afc77690b4cef2605420d67c1d1ceb28760cc

Modified Files
--------------
doc/src/sgml/func.sgml                       |  5 -----
src/backend/access/transam/xlogfuncs.c       |  8 ++------
src/test/regress/expected/misc_functions.out | 30 +++++++++++++++++++++++++++-
src/test/regress/sql/misc_functions.sql      | 15 +++++++++++++-
4 files changed, 45 insertions(+), 13 deletions(-)