Datagram UNIX sockets are not supported

[X512]

This is hrev56688.

Datagram (SOCK_DGRAM) UNIX sockets (AF_UNIX) are currently not supported. Only stream (SOCK_STREAM) UNIX sockets are currently working. The difference between datagram and stream UNIX sockets are that datagram sockets preserve packet boundaries but stream sockets don't.

Many software based on socket IPC (example: WebKit POSIX backend) require datagram sockets and don't work correctly with stream sockets.

Test code:

#include <sys/socket.h>
#include <stdio.h>

int main()
{
	int fd1 = socket(AF_UNIX, SOCK_DGRAM, 0);
	printf("fd1: %d\n", fd1);
	int fd2 = socket(AF_UNIX, SOCK_STREAM, 0);
	printf("fd2: %d\n", fd2);
	return 0;
}

Expected result:

fd1: 3
fd2: 4

Actual result:

fd1: -1
fd2: 3

Related code: ​https://git.haiku-os.org/haiku/tree/src/add-ons/kernel/network/protocols/unix

[waddlesplash]

Implemented in hrev57155.