examples: omit forward declarations, apply misc fixes

[vszakats]

• reorder functions to not need forward declarations.
• sync ephiperfifo.c and evhiperfifo.c.
• drop redundant casts for calloc() return value.
• ephiperfifo: silence unused variable warning.
• fix indent and apply clang-format more.

[bagder]

@vszakats my feedback on the clang format style used on evhiperfifo.c:

The leading comments should be indented by one space

-This is purely a demo app, all retrieved data is simply discarded by the write
-callback.
+ This is purely a demo app, all retrieved data is simply discarded by the write
+ callback.

Continued C expressions should be indented two steps:

   int action = ((revents & EV_READ) ? CURL_POLL_IN : 0) |
-               ((revents & EV_WRITE) ? CURL_POLL_OUT : 0);
+    ((revents & EV_WRITE) ? CURL_POLL_OUT : 0);

I'm not saying we should change this now. I just want to highlight that I don't think this clang format config is perfect.

[vszakats]

@vszakats my feedback on the clang format style used on evhiperfifo.c:

The leading comments should be indented by one space

-This is purely a demo app, all retrieved data is simply discarded by the write
-callback.
+ This is purely a demo app, all retrieved data is simply discarded by the write
+ callback.

I rarely let clang-format touch comments, and did not change these
in this PR. Also agree it's wrong as it is. But, I think single-space
indented comments isn't actually used in the source anywhere now
(after fixing a few outliers in a different PR just recently). Wouldn't this
rather be the one that matches the rest of code:

-This is purely a demo app, all retrieved data is simply discarded by the write
-callback.
+ * This is purely a demo app, all retrieved data is simply discarded by the write
+ * callback.

and have the * fence for all lines?

I used this to verify: git grep '^ [^ *]' ':**.c' ':**.h'

(only gipher.c has it, and another source, that I fixed locally last week,
but not pushed yet.)

Continued C expressions should be indented two steps:

   int action = ((revents & EV_READ) ? CURL_POLL_IN : 0) |
-               ((revents & EV_WRITE) ? CURL_POLL_OUT : 0);
+    ((revents & EV_WRITE) ? CURL_POLL_OUT : 0);

I'm not saying we should change this now. I just want to highlight that I don't think this clang format config is perfect.

Indeed, I gave in to clang-format in this case, because both were
already used in these sibling-examples, and went with its suggestion
to align them, and found this more readable, knowing that it's not the
preferred style. History had a little bit of both here, even within
the same file.

I'll be sticking to the other style.

[vszakats]

PR fixing the comments (and more, I had pending locally): #20361