conncheck: fix try-alternate on tcp turn sockets

In OC2007R2 compatibility mode, we observed the behaviour of a skype
turn server, when returning code 300 (try-alternate) stun error on its
tls connections. This value is returned apparently when the turn server
is overloaded already.

We noticed that the actual code in priv_handle_turn_alternate_server()
cannot handle a non-udp turn server, because a tcp one would require
to create a new socket.

But, even when creating such a new socket stack (tcp-bsd socket +
pseudossl socket), libnice still fails to establish a new connection to
the alternate server on port 443, in a very systematic way. I'm not sure
whether this problem is specific to this skype server infrastructure
(the skype client fails in a similar way). Anyway, this code path works
as expected with a non-microsoft turn server (tested with coturn).
11 jobs for master in 6 minutes and 22 seconds
Status Job ID Name Coverage
  Build
passed #2617238
build autotools

00:03:16

passed #2617239
build meson

00:00:39

failed #2617241
docker windows 1809
build msvc amd64

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failed #2617242
docker windows 1809
build msvc x86

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failed #2617240
docker windows 1809
build msys2

00:02:31

 
  Test
passed #2617245
distcheck autotools

00:02:35

passed #2617248
doc-and-install meson

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passed #2617243
test autotools

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passed #2617244
test autotools valgrind

00:03:05

passed #2617246
test meson

00:01:21

failed #2617247
test valgrind meson

00:01:13

 
Name Stage Failure
failed
test valgrind meson Test
==380== For counts of detected and suppressed errors, rerun with: -v
==380== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 23 from 23)
-------

Full log written to /builds/libnice/libnice/build/meson-logs/testlog-valgrind.txt
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id=2617247 responseStatus=201 Created token=Wbh4MZCn
ERROR: Job failed: exit code 1
failed
build msys2 Build There has been a runner system failure, please try again
downloading mingw-w64-x86_64-tools-git-8.0.0.5813.223de3ab-1-any.pkg.tar.xz...
downloading mingw-w64-x86_64-winstorecompat-git-8.0.0.5813.223de3ab-1-any.pkg.tar.xz...
downloading ninja-1.10.0-1-x86_64.pkg.tar.xz...
checking keyring...
checking package integrity...
loading package files...
checking for file conflicts...
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failed
build msvc x86 Build There has been a runner system failure, please try again
|Header <sys/types.h> has symbol "major" : NO 
|Header <dlfcn.h> has symbol "RTLD_LAZY" : NO
|Header <dlfcn.h> has symbol "RTLD_NOW" : NO
|Header <dlfcn.h> has symbol "RTLD_GLOBAL" : NO
|Header <dlfcn.h> has symbol "RTLD_NEXT" : NO
|Message: Checking whether to use statfs or statvfs .. neither
|Checking if "futex(2) system call" links: NO
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ERROR: Job failed (system failure): aborted: <nil>
failed
build msvc amd64 Build There has been a runner system failure, please try again
  File "c:\python38\lib\site-packages\mesonbuild\interpreterbase.py", line 212, in wrapped
return f(*wrapped_args, **wrapped_kwargs)
File "c:\python38\lib\site-packages\mesonbuild\interpreter.py", line 1210, in symbols_have_underscore_prefix_method
return self.compiler.symbols_have_underscore_prefix(self.environment)
File "c:\python38\lib\site-packages\mesonbuild\compilers\mixins\clike.py", line 797, in symbols_have_underscore_prefix
raise RuntimeError(m.format(n, p.stdo))
RuntimeError: BUG: Unable to compile 'symbols_have_underscore_prefix' check:
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ERROR: Job failed (system failure): aborted: <nil>