configure: silence compiler warnings in feature checks, drop duplicates

[vszakats]

Silence compiler warnings (200 of them across the main CI workflows):

warning #2193: null argument provided for parameter marked with attribute "nonnull"
warning: Null pointer passed to 1st parameter expecting 'nonnull' [core.NonNullParamChecker]
warning: Null pointer passed to 2nd parameter expecting 'nonnull' [core.NonNullParamChecker]
warning: argument 1 null where non-null expected [-Wnonnull]
warning: argument 2 null where non-null expected [-Wnonnull]
warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits]
warning: null argument where non-null required (argument 1) [-Wnonnull]

Also drop if ... can be linked feature checks that were identical to
if ... is compilable checks, for:
closesocket, ioctlsocket, socket, freeaddrinfo, getaddrinfo,
gethostname, getpeername, getsockname,
CloseSocket (AmigaOS), IoctlSocket (AmigaOS).
Another option is to really do the link checks. But, if they weren't
missed so far, it seems safer to drop than risk a detection failure,
as was the case with AmigaOS functions while working on this PR.

There remain 22 -Wnonnull warnings in gethostbyname_r(),
getpeername() getsockname(). Most of the rest is necessary for
detection, or originate from autotools and CMake detection code
templates. Some still fixable, like duplicate libs.

Follow-up to ca2f49d #16287

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w/o ws https://github.com/curl/curl/pull/16377/files?w=1

This may be of interest, also in tests/server/util.c where the deprecated siginterrupt serves as a fallback
since 3fb6e5a:

warning #1786: function "siginterrupt" (declared at line 324 of "/usr/include/signal.h") was declared deprecated ("Use sigaction with SA_RESTART instead")
warning: 'siginterrupt' is deprecated: Use sigaction with SA_RESTART instead [-Wdeprecated-declarations]