Stub getpwnam(), setuid() and setgid()

Implement getpwname(), setuid() and setgid() in the simplest way possible
considering that we don't support any userid except 0:

getpwname() returns user 0 for any username given to it.
setuid() and setgid() does nothing for uid or gid 0, otherwise fails.
Where would the caller get this !=0 id anyway?

Memcached needs these calls, because it wants to be clever and
warn the user against running it as root....

libc/build.mk

@@ -634,3 +634,4 @@ libc += sem.o
 libc += pipe_buffer.o
 libc += pipe.o
 libc += af_local.o
+libc += user.o

libc/user.cc

+// OSV does not support different users, and getuid(), getgid() and friends
+// always return 0.
+// In this file we implement various user-handling functions in traditional
+// libc, in the most sensible way we can given our limitations. In particular:
+// 1. Every user name resolves to user id 0.
+// 2. setuid() to any other user id but 0 fails on assertion
+
+#include <sys/types.h>
+#include <pwd.h>
+#include <unistd.h>
+
+#include <debug.hh>
+
+static char username[] = "osv";
+static char password[] = "";
+static char gecos[] = "OSV User";
+static char homedir[] = "/";
+static char shell[] = "?";
+
+static struct passwd single_user = {
+        username, password, 0, 0, gecos, homedir, shell
+};
+
+struct passwd *getpwnam(const char *name)
+{
+   return &single_user;
+}
+
+int setuid(uid_t uid)
+{
+    assert(uid == 0);
+    return 0;
+}
+
+int setgid(gid_t gid)
+{
+    assert(gid == 0);
+    return 0;
+}