curl: show the libcurl release date in --version output

... and support and additional "security patched" date for those who
enhance older versions that way. Pass on the define CURL_PATCHSTAMP with
a date for that.

Building with non-release headers shows the date as [unreleased].

Also: this changes the date format generated in the curlver.h file to be
"YYYY-MM-DD" (no name of the day or month, no time, no time zone) to
make it easier on the eye and easier to parse. Example (new) date
string: 2017-05-09

Suggested-by: Brian Childs

Closes #1474