non-blocking mode, and decouple function detection from function capability.

 and CHECK_FUNC_GETADDRINFO_UNFREEABLE_AI_CANONNAME
internal to CHECK_FUNC_GETADDRINFO

function returns an addrinfo with an unfreeable ai_canonname member ptr.

function returns an addrinfo with an unfreeable ai_addr member ptr.

configure is unable of finding out if it has glibc-style or POSIX-style,
trying to ensure that this condition does not go unnoticed.

inet_pton function is available.

and when not crosscompiling verifies if it is IPv6 capable.

HAVE_INET_NTOP will only be defined when an IPv6 capable working
inet_ntop function is available.

check for winsock build target configurations

that lacks the writev function.

  strcasecmp()
  strcasestr()
  strcmpi()
  stricmp()
  strlcat()
  strncasecmp()
  strncmpi()
  strnicmp()