BUG #15141: Faulty ISO 8601 parsing

The following bug has been logged on the website:

Bug reference:      15141
Logged by:          defanor
Email address:      defanor@uberspace.net
PostgreSQL version: 10.0
Operating system:   Any, apparently
Description:

The time parsing fails on some valid ISO times, with some locales, e.g.:

# select to_timestamp('2018-04-03T01:45:00,728456785+0000')::timestamp with
time zone;
ERROR:  invalid input syntax for type double precision:
"2018-04-03T01:45:00,728456785+0000"
LINE 1: select to_timestamp('2018-04-03T01:45:00,728456785+0000')::t...
                            ^

Apparently the parsing is locale-dependent (using the locale-dependent
strtod function), while ISO 8601 permits both comma and full stop, with a
preference for comma (and without mentioning locales). Would be nice to
handle both, so that any valid ISO times would get parsed.