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

1 files changed, 6 insertions, 0 deletions

diff --git a/src/core/execute.h b/src/core/execute.h
index 189c4d0999..48cc18fbb3 100644
--- a/src/core/execute.h
+++ b/src/core/execute.h
@@ -92,6 +92,8 @@ struct ExecRuntime {
         char *tmp_dir;
         char *var_tmp_dir;
 
+        /* An AF_UNIX socket pair, that contains a datagram containing a file descriptor referring to the network
+         * namespace. */
         int netns_storage_socket[2];
 };
 
@@ -174,6 +176,8 @@ struct ExecContext {
 
         bool no_new_privileges;
 
+        bool dynamic_user;
+
         /* This is not exposed to the user but available
          * internally. We need it to make sure that whenever we spawn
          * /usr/bin/mount it is run in the same process group as us so
@@ -235,12 +239,14 @@ struct ExecParameters {
 };
 
 #include "unit.h"
+#include "dynamic-user.h"
 
 int exec_spawn(Unit *unit,
                ExecCommand *command,
                const ExecContext *context,
                const ExecParameters *exec_params,
                ExecRuntime *runtime,
+               DynamicCreds *dynamic_creds,
                pid_t *ret);
 
 void exec_command_done(ExecCommand *c);