Drop autotools option `--disable-clear-memory`, CMake option
`-DCLEAR_MEMORY=NO`.
Keep this option always enabled. If there is not API support, use the
local fallback.
Introduce a temporary CPPFLAG to disable system memory clearing
functions, in case they cause a build issue.
Follow-up to a0e424a51c#810Closes#2454
Checker script and wordlist comes from curl.
Also:
- checksrc.pl: sync up with curl.
- unfold an IETF URL.
- update all IETF URLs where missing.
Closes#1959