not always zero!

Reintroduce checking for HAVE_MSG_NOSIGNAL in configure script, so that we don't depend on header inclusion order for a valid check.

Simplify check for NEED_MALLOC_H, and make more explicit that NEED_MALLOC_H shall be defined if <malloc.h> header file must be included even when including <stdlib.h>.

Define NEED_MALLOC_H if including <stdlib.h> is not enough for proper compilation and <malloc.h> must also be included.

Finally get rid of CURL_CHECK_HEADERS_ONCE since it adds very little value and has portability issues.

Change some shell if...then...fi tests into case...esac tests which demand less resources.

Using backslashes and slashes in the strings of the sed 'y' command shall be avoided since its interpretation is not the same across platforms.

Now we use the sed 's' command with a bracket expression.

Prevent definition of HAVE_WINxxx_H symbols and avoid inclusion of Windows headers when compiled with Cygwin in POSIX emulation mode.

HAVE_GETNAMEINFO, GETNAMEINFO_QUAL_ARG1, GETNAMEINFO_TYPE_ARG1, GETNAMEINFO_TYPE_ARG2, GETNAMEINFO_TYPE_ARG46, GETNAMEINFO_TYPE_ARG7

HAVE_RECV, RECV_TYPE_ARG1, RECV_TYPE_ARG2, RECV_TYPE_ARG3, RECV_TYPE_ARG4, RECV_TYPE_RETV

HAVE_SEND, SEND_TYPE_ARG1, SEND_TYPE_ARG2, SEND_TYPE_ARG3, SEND_TYPE_ARG4, SEND_TYPE_RETV