[HTTP Strict Transport Security](https://tools.ietf.org/html/rfc6797)
## Behavior
libcurl features an in-memory cache for HSTS hosts, so that subsequent
HTTP-only requests to a host name present in the cache will get internally
"redirected" to the HTTPS version.
## `curl_easy_setopt()` options:
-`CURLOPT_HSTS_CTRL` - enable HSTS for this easy handle
-`CURLOPT_HSTS` - specify file name where to store the HSTS cache on close
(and possibly read from at startup)
## curl cmdline options
-`--hsts [filename]` - enable HSTS, use the file as HSTS cache. If filename
is `""` (no length) then no file will be used, only in-memory cache.
## HSTS cache file format
Lines starting with `#` are ignored.
For each hsts entry:
[host name] "YYYYMMDD HH:MM:SS"
The `[host name]` is dot-prefixed if it is a includeSubDomain.
The time stamp is when the entry expires.
I considered using wget's file format for the HSTS cache. However, they store the time stamp as the epoch (number of seconds since 1970) and I strongly disagree with using that format. Instead I opted to use a format similar to the curl alt-svc cache file format.
## Possible future additions
-`CURLOPT_HSTS_PRELOAD` - provide a set of preloaded HSTS host names
