Handle \v as a whitespace character in parsers

Handle \v as a whitespace character in parsers

This commit comes as a continuation of the discussion that has led to
d522b05, as \v was handled inconsistently when parsing array values or
anything going through the parsers, and changing a parser behavior in
stable branches is a scary thing to do.  The parsing of array values now
uses the more central scanner_isspace() and array_isspace() is removed.

As pointing out by Peter Eisentraut, fix a confusing reference to
horizontal space in the parsers with the term "horiz_space".  \f was
included in this set since 3cfdd8f from 2000, but it is not horizontal.
"horiz_space" is renamed to "non_newline_space", to refer to all
whitespace characters except newlines.

The changes impact the parsers for the backend, psql, seg, cube, ecpg
and replication commands.  Note that JSON should not escape \v, as per
RFC 7159, so these are not touched.

Reviewed-by: Peter Eisentraut, Tom Lane
Discussion: https://postgr.es/m/ZJKcjNwWHHvw9ksQ@paquier.xyz