core: add a concept of "dynamic" user ids, that are allocated as long as a service is running

This adds a new boolean setting DynamicUser= to service files. If set, a new
user will be allocated dynamically when the unit is started, and released when
it is stopped. The user ID is allocated from the range 61184..65519. The user
will not be added to /etc/passwd (but an NSS module to be added later should
make it show up in getent passwd).

For now, care should be taken that the service writes no files to disk, since
this might result in files owned by UIDs that might get assigned dynamically to
a different service later on. Later patches will tighten sandboxing in order to
ensure that this cannot happen, except for a few selected directories.

A simple way to test this is:

        systemd-run -p DynamicUser=1 /bin/sleep 99999

2 files changed, 16 insertions, 0 deletions

diff --git a/src/basic/socket-util.c b/src/basic/socket-util.c
index 385c3e4df3..6093e47172 100644
--- a/src/basic/socket-util.c
+++ b/src/basic/socket-util.c
@@ -1046,3 +1046,17 @@ int flush_accept(int fd) {
                 close(cfd);
         }
 }
+
+struct cmsghdr* cmsg_find(struct msghdr *mh, int level, int type, socklen_t length) {
+        struct cmsghdr *cmsg;
+
+        assert(mh);
+
+        CMSG_FOREACH(cmsg, mh)
+                if (cmsg->cmsg_level == level &&
+                    cmsg->cmsg_type == type &&
+                    (length == (socklen_t) -1 || length == cmsg->cmsg_len))
+                        return cmsg;
+
+        return NULL;
+}
diff --git a/src/basic/socket-util.h b/src/basic/socket-util.h
index e9230e4a9f..2536b085f9 100644
--- a/src/basic/socket-util.h
+++ b/src/basic/socket-util.h
@@ -142,6 +142,8 @@ int flush_accept(int fd);
 #define CMSG_FOREACH(cmsg, mh)                                          \
         for ((cmsg) = CMSG_FIRSTHDR(mh); (cmsg); (cmsg) = CMSG_NXTHDR((mh), (cmsg)))
 
+struct cmsghdr* cmsg_find(struct msghdr *mh, int level, int type, socklen_t length);
+
 /* Covers only file system and abstract AF_UNIX socket addresses, but not unnamed socket addresses. */
 #define SOCKADDR_UN_LEN(sa)                                             \
         ({                                                              \