lib: provide a getaddrinfo wrapper

This uses c-ares under the hood and supports the CURL_DNS_SERVER
environment variable - for debug builds only. The getaddrinfo()
replacement function is only used if CURL_DNS_SERVER is set to make a
debug build work more like a release version without the variable set.

'override-dns' is a new feature for the test suite when curl can be told
to use a dedicated DNS server, and test 2102 is the first to require
this.

Requires c-ares 1.26.0 or later.

Closes #17134

lib/Makefile.inc

@@ -123,10 +123,10 @@ LIB_CFILES =         \
   cf-socket.c        \
   cfilters.c         \
   conncache.c        \
-  cshutdn.c          \
   connect.c          \
   content_encoding.c \
   cookie.c           \
+  cshutdn.c          \
   curl_addrinfo.c    \
   curl_des.c         \
   curl_endian.c      \
@@ -154,6 +154,7 @@ LIB_CFILES =         \
   easygetopt.c       \
   easyoptions.c      \
   escape.c           \
+  fake_addrinfo.c    \
   file.c             \
   fileinfo.c         \
   fopen.c            \
@@ -304,6 +305,7 @@ LIB_HFILES =         \
   easyif.h           \
   easyoptions.h      \
   escape.h           \
+  fake_addrinfo.h    \
   file.h             \
   fileinfo.h         \
   fopen.h            \