Use more accurate names for the date conversion functions

Use more accurate names for the date conversion functions

The new names are epochToRssDate and strToRssDate.

Don't refer to gmdate() in the function names, this is just an
implementation detail which should not have leaked into the external
interface, instead mention RssDate in the function names to communicate
something about the output they produce.

Also, while at it, user the DATE_RSS format when calling gmdate().