Rename pg_dissect_walfile_name() to pg_split_walfile_name()

Rename pg_dissect_walfile_name() to pg_split_walfile_name()

The former name was discussed as being confusing, so use "split", as per
a suggestion from Magnus Hagander.

While on it, one of the output arguments is renamed from "segno" to
"segment_number", as per a suggestion from Kyotaro Horiguchi.

The documentation is updated to reflect all these changes.

Bump catalog version.

Author: Bharath Rupireddy, Michael Paquier
Discussion: https://postgr.es/m/CABUevEytQVaOOhGdoh0D7hGwe3fuKcRF6NthsSW7ww04EmtFgQ@mail.gmail.com