os2: initial implementation for Darwin&BSDs, process API is only thing incomplete

core/os/os2/errors_posix.odin

@@ -0,0 +1,30 @@
+//+private
+//+build darwin, netbsd, freebsd, openbsd
+package os2
+
+import "core:sys/posix"
+
+_error_string :: proc(errno: i32) -> string {
+	return string(posix.strerror(posix.Errno(errno)))
+}
+
+_get_platform_error :: proc() -> Error {
+	#partial switch errno := posix.errno(); errno {
+	case .EPERM:
+		return .Permission_Denied
+	case .EEXIST:
+		return .Exist
+	case .ENOENT:
+		return .Not_Exist
+	case .ETIMEDOUT:
+		return .Timeout
+	case .EPIPE:
+		return .Broken_Pipe
+	case .EBADF:
+		return .Invalid_File
+	case .ENOMEM:
+		return .Out_Of_Memory
+	case:
+		return Platform_Error(errno)
+	}
+}