amigaos: fix threaded resolver on AmigaOS 4.x

[Futaura]

Replace ip4 resolution function on AmigaOS 4.x, as it requires
runtime feature detection and extra code to make it thread safe.

[bagder]

statics really? Are you sure these are thread-safe?

[Futaura]

I'm with you on the statics, but this is the only way I could think of, without forcing unnecessary extra overhead on every resolve. Normally, I'd rely on the global ISocket variable which the application will have (if using AmiSSL), but since I need to enable the SBTC_CAN_SHARE_LIBRARY_BASES on it to make it thread safe, I didn't want to interfere with that, or indeed allow the application to interfere with libcurl by switching that feature off!

Curl_amiga_init() isn't thread safe by itself, when it comes to initialising __CurlISocket, but as long as GLOBAL_INIT_IS_THREADSAFE is defined it will be thread safe, as it will be on AmigaOS 4.x builds - that should be enough, right? Once initialised, the functionality is completely thread safe (the old code wasn't as it relied on gethostbyname().

This code is going to get reused for enabling getaddrinfo() support to in the future, but that function appears to be broken in the AmigaOS 4.x tcp/ip stack at the moment!

[bagder]

This PR appears to have merge conflicts now. Can you please rebase and force-push it?

[bagder]

It still says "This branch cannot be rebased due to conflicts". I really did mean rebase and force-push...

[Futaura]

Sorry - was assuming the buttons on the GitHub website would help me out, but seemed to created a bigger mess! Having booted up linux, hopefully my first attempt at a rebase is correct, but please let me know if not. Thanks.

[bagder]

Thanks!

[gvanem]

Real retro stuff: curl on Amiga. (just a test of HTML and web-fonts).