pv-wrap: Communicate timeouts to pv-adverb as locale-independent ASCII

These arguments take a G_OPTION_ARG_DOUBLE, which is documented to be parsed using either the user's locale or the C locale (in fact they use g_strtod(), which parses it both ways and takes the longer match). The locale matters in locales like de_DE that use a decimal comma instead of a decimal point: we can always parse 2.000000 in the C locale, but we can only parse 2,000000 if we are in a correctly-set-up locale that uses the decimal comma.

If we needed to generate new locale files inside the container, then pv-adverb needs to be able to parse its command-line before we have generated those locale files. This means that --terminate-idle-timeout=2,000000 won't work reliably, so we need to use the C locale format, --terminate-idle-timeout=2.000000.

Using g_ascii_dtostr() has the side benefit that it uses the shortest possible representation that does not lose precision; in the common case that the timeout is an integer number of seconds, we'll just print it as an integer.

Helps: https://github.com/ValveSoftware/steam-runtime/issues/381