- replace tabs with spaces where possible
- remove line ending spaces
- remove double/triple newlines at EOF
- fix a non-UTF-8 character
- cleanup a few indentations/line continuations
  in manual examples

Closes https://github.com/curl/curl/pull/3037

All plain C examples now (mostly) adhere to the curl code style. While
they are only examples, they had diverted so much and contained all
sorts of different mixed code styles by now. Having them use a unified
style helps users and readability. Also, as they get copy-and-pasted
widely by users, making sure they're clean and nice is a good idea.

573 checksrc warnings were addressed.

Only <curl/curl.h> is needed typically and curl/types.h has been removed

to curl_easy_setopt instead of long.

example programs.

These minor changes remove portability issues with the this example and allow
it to run on Win32. Specifically:

* The use of pthread_create() has been replaced by g_thread_create(). This
removes the dependency on the pthreads library. Since this is an example using
GTK+, g_thread_create() is available as it is a part of glibc.

* The CURLOPT_FILE option is now referred to by its "newer name"
CURLOPT_WRITEDATA.

* The use of CURLOPT_WRITEFUNCTION has been added.  As described in the docs,
this avoids the crashes when using a DLL under Win32.

* The output file has been renamed from "/tmp/test.curl" to "test.curl". It's
unlikely that there is a /tmp when in Win32 and other examples in libcurl
write their output files to the working directory.