unix: use the presence of SOCK_* instead of OS macros for socketpair

[panjf2000]

Stuffing SOCK_NONBLOCK and SOCK_CLOEXEC into the type argument when calling socketpair() is widely supported across UNIX-like OS: Linux, BSD, Solaris, etc., as is the socket(). socket() and socketpair() share the SOCK_ flags under the hood.

Currently, we detect the presence of SOCK_NONBLOCK and SOCK_CLOEXEC for socket():

libuv/src/unix/core.c

Lines 494 to 499 in bf61390

	int uv__socket(int domain, int type, int protocol) {
	int sockfd;
	int err;
	#if defined(SOCK_NONBLOCK) && defined(SOCK_CLOEXEC)
	sockfd = socket(domain, type | SOCK_NONBLOCK | SOCK_CLOEXEC, protocol);

while checking the specific OS macros for socketpair(). We keep them in sync to expand the scope of SOCK_* flags for socketpair().

References

• socketpair(2) on Linux
• socketpair(2) on FreeBSD
• socketpair(2) on DragonFly
• socketpair(2) on NetBSD
• socketpair(2) on OpenBSD
• socketpair(3C) on Solaris

[panjf2000]

Hi @saghul, shall we move forward with this PR?

[saghul]

Nice work!