http: ignore invalid Retry-After times

- Treat negative Retry-After date-based times as 0.

- Treat Retry-After times greater than 6 hours as 6 hours.

Prior to this change Retry-After did not have a limited range and the
server could have set a time greater than 6 hours or a date in the past
that would result in a negative time, either of which may be unexpected
by the user.

The 6 hour limit is purposely not documented so that it can be changed
in the future if necessary.

Closes https://github.com/curl/curl/pull/15833

tests/data/test1596

@@ -13,9 +13,9 @@ If-Modified-Since
 <reply>
 <data nocheck="yes">
 HTTP/1.1 429 Too Many Requests
-Date: Thu, 11 Jul 2019 02:26:59 GMT
+Date: Wed, 31 Dec 2036 02:26:59 GMT
 Server: test-server/swsclose
-Retry-After: Thu, 11 Jul 2024 02:26:59 GMT
+Retry-After: Wed, 31 Dec 2036 02:26:59 GMT
 
 </data>
 </reply>
@@ -42,8 +42,9 @@ Host: %HOSTIP:%HTTPPORT
 Accept: */*
 
 </protocol>
+# Retry-After time is limited to 6 hours (21600 seconds)
 <stdout>
-Retry-After 172066
+Retry-After 21600
 </stdout>
 </verify>
 </testcase>