execute: Add new PassEnvironment= directive

This directive allows passing environment variables from the system
manager to spawned services. Variables in the system manager can be set
inside a container by passing `--set-env=...` options to systemd-spawn.

Tested with an on-disk test.service unit. Tested using multiple variable
names on a single line, with an empty setting to clear the current list
of variables, with non-existing variables.

Tested using `systemd-run -p PassEnvironment=VARNAME` to confirm it
works with transient units.

Confirmed that `systemctl show` will display the PassEnvironment
settings.

Checked that man pages are generated correctly.

No regressions in `make check`.

src/basic/env-util.c

@@ -138,6 +138,21 @@ bool strv_env_is_valid(char **e) {
         return true;
 }
 
+bool strv_env_name_is_valid(char **l) {
+        char **p, **q;
+
+        STRV_FOREACH(p, l) {
+                if (!env_name_is_valid(*p))
+                        return false;
+
+                STRV_FOREACH(q, p + 1)
+                        if (streq(*p, *q))
+                                return false;
+        }
+
+        return true;
+}
+
 bool strv_env_name_or_assignment_is_valid(char **l) {
         char **p, **q;
 

src/basic/env-util.h

@@ -36,6 +36,7 @@ bool strv_env_is_valid(char **e);
 #define strv_env_clean(l) strv_env_clean_with_callback(l, NULL, NULL)
 char **strv_env_clean_with_callback(char **l, void (*invalid_callback)(const char *p, void *userdata), void *userdata);
 
+bool strv_env_name_is_valid(char **l);
 bool strv_env_name_or_assignment_is_valid(char **l);
 
 char **strv_env_merge(unsigned n_lists, ...);