libsystemd: new library, refactor sd-daemon.[ch] test code

This commit introduces a bare-bones replacement for libsystemd:

 - Build .so file and add --with-libsystemd to configure
 - Add capabilities support to test/src/serv.c
 - Update tests to account for a Finit built w/o libsystemd support

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

libsystemd/sd-daemon.c

@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: MIT-0 */
+
+/*
+ * systemd notify protocol, supporting readiness notification on startup
+ * and reloading, according to the protocol defined at:
+ * https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
+ *
+ * This protocol is guaranteed to be stable as per:
+ * https://systemd.io/PORTABILITY_AND_STABILITY/
+ */
+
+#define _GNU_SOURCE 1
+#include <errno.h>
+#include <inttypes.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <time.h>
+#include <unistd.h>
+
+/*
+ * NOTE: As per spec., this function does not return POSIX OK(0) but
+ *       rather "a positive value", i.e., 1.
+ */
+int sd_notify(int unset_environment, const char *state)
+{
+	struct sockaddr_un sun = {
+		.sun_family = AF_UNIX,
+	};
+	size_t len, sun_len;
+	const char *path;
+	ssize_t written;
+	int sd;
+
+	if (!state)
+		return -EINVAL;
+
+	len = strlen(state);
+	if (len == 0)
+		return -EINVAL;
+
+	/* If the variable is not set, the protocol is a noop */
+	path = getenv("NOTIFY_SOCKET");
+	if (!path)
+		return 0;	/* Not set? Nothing to do */
+
+	/* Only AF_UNIX is supported, with path or abstract sockets */
+	if (path[0] != '/' && path[0] != '@')
+		return -EAFNOSUPPORT;
+
+	/* Ensure there is room for NUL byte */
+	sun_len = strlen(path);
+	if (sun_len >= sizeof(sun.sun_path))
+		return -E2BIG;
+
+	strlcpy(sun.sun_path, path, sizeof(sun.sun_path));
+	if (sun.sun_path[0] == '@')
+		sun.sun_path[0] = 0;
+
+	sd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+	if (sd == -1)
+		return -errno;
+
+	if (connect(sd, (struct sockaddr *)&sun, offsetof(struct sockaddr_un, sun_path) + sun_len) == -1) {
+		close(sd);
+		return -errno;
+	}
+
+	if (unset_environment)
+		unsetenv("NOTIFY_SOCKET");
+
+	written = write(sd, state, len);
+	if (written != (ssize_t) len) {
+		close(sd);
+		return written < 0 ? -errno : -EPROTO;
+	}
+
+	return 1;
+}