Closes #6172

The OS is deprecated. I see no traces of anyone having actually built
curl for Symbian after 2012.

The public headers are unmodified.

Closes #5989

These files are not maintained, they seem to have no users, Watcom
compilers look like not having users nor releases anymore.

Closes #5918

... we're generally bad at this, but we are adding new functions for
this release.

Closes #5899

 const struct curl_easyoption *curl_easy_option_by_name(const char *name);

 const struct curl_easyoption *curl_easy_option_by_id (CURLoption id);

 const struct curl_easyoption *
 curl_easy_option_next(const struct curl_easyoption *prev);

The purpose is to provide detailed enough information to allow for
example libcurl bindings to get option information at run-time about
what easy options that exist and what arguments they expect.

Assisted-by: Jeroen Ooms
Closes #5365

Follow up to d820224b8b

Repeatedly we see problems where using curl_multi_wait() is difficult or
just awkward because if it has no file descriptor to wait for
internally, it returns immediately and leaves it to the caller to wait
for a small amount of time in order to avoid occasional busy-looping.

This is often missed or misunderstood, leading to underperforming
applications.

This change introduces curl_multi_poll() as a replacement drop-in
function that accepts the exact same set of arguments. This function
works identically to curl_multi_wait() - EXCEPT - for the case when
there's nothing to wait for internally, as then this function will by
itself wait for a "suitable" short time before it returns. This
effectiely avoids all risks of busy-looping and should also make it less
likely that apps "over-wait".

This also changes the curl tool to use this funtion internally when
doing parallel transfers and changes curl_easy_perform() to use it
internally.

Closes #4163