core: add new per-unit setting KeyringMode= for controlling kernel keyring setup

Usually, it's a good thing that we isolate the kernel session keyring
for the various services and disconnect them from the user keyring.
However, in case of the cryptsetup key caching we actually want that
multiple instances of the cryptsetup service can share the keys in the
root user's user keyring, hence we need to be able to disable this logic
for them.

This adds KeyringMode=inherit|private|shared:

    inherit: don't do any keyring magic (this is the default in systemd --user)
    private: a private keyring as before (default in systemd --system)
    shared: the new setting

src/core/execute.h

@@ -80,6 +80,14 @@ typedef enum ExecPreserveMode {
         _EXEC_PRESERVE_MODE_INVALID = -1
 } ExecPreserveMode;
 
+typedef enum ExecKeyringMode {
+        EXEC_KEYRING_INHERIT,
+        EXEC_KEYRING_PRIVATE,
+        EXEC_KEYRING_SHARED,
+        _EXEC_KEYRING_MODE_MAX,
+        _EXEC_KEYRING_MODE_INVALID = -1,
+} ExecKeyringMode;
+
 struct ExecStatus {
         dual_timestamp start_timestamp;
         dual_timestamp exit_timestamp;
@@ -189,6 +197,8 @@ struct ExecContext {
         bool smack_process_label_ignore;
         char *smack_process_label;
 
+        ExecKeyringMode keyring_mode;
+
         char **read_write_paths, **read_only_paths, **inaccessible_paths;
         unsigned long mount_flags;
         BindMount *bind_mounts;
@@ -368,5 +378,8 @@ ExecUtmpMode exec_utmp_mode_from_string(const char *s) _pure_;
 const char* exec_preserve_mode_to_string(ExecPreserveMode i) _const_;
 ExecPreserveMode exec_preserve_mode_from_string(const char *s) _pure_;
 
+const char* exec_keyring_mode_to_string(ExecKeyringMode i) _const_;
+ExecKeyringMode exec_keyring_mode_from_string(const char *s) _pure_;
+
 const char* exec_directory_type_to_string(ExecDirectoryType i) _const_;
 ExecDirectoryType exec_directory_type_from_string(const char *s) _pure_;