add setxid.c for new set*id() framework. missed in last commit.

src/unistd/setxid.c

+#include <unistd.h>
+#include <errno.h>
+#include <sys/resource.h>
+#include "syscall.h"
+#include "libc.h"
+
+struct ctx {
+	int id, eid, sid;
+	int nr, rlim, err;
+};
+
+/* We jump through hoops to eliminate the possibility of partial failures. */
+
+static void do_setxid(void *p)
+{
+	struct ctx *c = p;
+	if (c->err) return;
+	if (c->rlim && c->id >= 0 && c->id != getuid()) {
+		struct rlimit inf = { RLIM_INFINITY, RLIM_INFINITY }, old;
+		getrlimit(RLIMIT_NPROC, &old);
+		if (setrlimit(RLIMIT_NPROC, &inf) && libc.threads_minus_1) {
+			c->err = errno;
+			return;
+		}
+		if (__syscall(c->nr, c->id, c->eid, c->sid))
+			c->err = errno;
+		setrlimit(RLIMIT_NPROC, &old);
+		return;
+	}
+	if (__syscall(c->nr, c->id, c->eid, c->sid))
+		c->err = errno;
+}
+
+int __setxid(int nr, int id, int eid, int sid)
+{
+	struct ctx c = { .nr = nr, .id = id, .eid = eid, .sid = sid };
+	switch (nr) {
+	case SYS_setuid:
+	case SYS_setreuid:
+	case SYS_setresuid:
+		c.rlim = 1;
+	}
+	__synccall(do_setxid, &c);
+	if (c.err) {
+		errno = c.err;
+		return -1;
+	}
+	return 0;
+}