Reported-by: Alain Miniussi
Fixes #5096

urlapi: turn three local-only functions into statics

conncache: make conncache_find_first_connection static

multi: make detach_connnection static

connect: make getaddressinfo static

curl_ntlm_core: make hmac_md5 static

http2: make two functions static

http: make http_setup_conn static

connect: make tcpnodelay static

tests: make UNITTEST a thing to mark functions with, so they can be static for
normal builds and non-static for unit test builds

... and mark Curl_shuffle_addr accordingly.

url: make up_free static

setopt: make vsetopt static

curl_endian: make write32_le static

rtsp: make rtsp_connisdead static

warnless: remove unused functions

memdebug: remove one unused function, made another static

On Windows, casting between unrelated function types is fine and
sometimes even necessary, so just use an intermediate cast to
(void (*) (void)) to silence the warning as described in [0].

[0] https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html

Closes https://github.com/curl/curl/pull/2860

unit1309 and vtls/gtls: error: arithmetic on a null pointer treated as a
cast from integer to pointer is a GNU extension

Reported-by: Rikard Falkeborn

Fixes #2466
Closes #2468

When targeting x64, MinGW-w64 complains about conversions between
32-bit long and 64-bit pointers. Fix this by reusing the
GNUTLS_POINTER_TO_SOCKET_CAST / GNUTLS_SOCKET_TO_POINTER_CAST logic
from gtls.c, moving it to warnless.h as CURLX_POINTER_TO_INTEGER_CAST /
CURLX_INTEGER_TO_POINTER_CAST.

Closes https://github.com/curl/curl/pull/2341

Not picked up by checksrc or Visual Studio but my own code review, this
would haven broken Intel based Unix builds - Perhaps I should learn to
type on my laptop's keyboard before committing!

There is an implicit conversion from "unsigned long" to "long"