Use macros from xlog_internal.h for WAL segment logic in pg_resetwal

Use macros from xlog_internal.h for WAL segment logic in pg_resetwal

When scanning for the end of WAL, pg_resetwal has been maintaining its
own internal logic to calculate segment numbers and to parse a WAL
segment name for its timeline and segment number.  The code claimed for
example that XLogFromFileName() cannot be used because it lacks the
possibility of specifying a WAL segment size, which is not the case
since fc49e24, that has made the WAL segment size configurable at
initialization time, extending this routine to do so.

Similarly, this switches one segment number calculation to use
XLByteToSeg() rather than the same logic present in xlog_internal.h.
While on it, switch to TimeLineID in pg_resetwal.c for TLI numbers
parsed from segment names, to be more consistent with
XLogFromFileName().  The maths are exactly the same, but the code gets
simplified.

Author: Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CALj2ACX+E_jnwqH_jmjhNG8BczJTNRTOLpw8K1CB1OcB48MJ8w@mail.gmail.com