use fetch_*() for I/O with SMALLPROG / !WITH_SSL builds

Adapt the SMALLPROG / -UWITH_SSL build to also use the fetch_*()
methods from ssl.c, instead of using stdio, as stdio isn't robust
when using interruptable signals.

Disable ssl-specific support in the fetch_*() methods if WITH_SSL
isn't defined, so SMALLPROG still doesn't have ssl support (as expected).

The resulting SMALLPROG binary is slightly larger than before
(e.g., 157KiB vs 153KiB on amd64).

Set version to 20210603 for this fix and the SO_KEEPALIVE fix for PR 56129.

PR install/56219

Make fetch_read() return size_t like fread() does. It is bogus to
have one backing implementation that returns different values and
types than the other. Handle error setting properly; i.e. bail
out if the internal read returned an error. Now we get a proper
error message when the the server resets our connection instead of
a warning that the right failed with an invalid argument.

The server used for testing was:
http://capeweather.dyndns.org:8080/graphs/3474.png
Which seems to be unreliable :-)

branches: 1.3.16;
Add Server Name Indication (SNI) support for https.

Needed for e.g. some github URLs.

branches: 1.2.4;
Annotate functions using format strings.

branches: 1.1.2; 1.1.6; 1.1.10;
PR/47276: Add https support