rework systemd's own process environment handling/passing

Stop importing non-sensical kernel-exported variables. All
parameters in the kernel command line are exported to the
initial environment of PID1, but suppressed if they are
recognized by kernel built-in code. The EFI booted kernel
will add further kernel-internal things which do not belong
into userspace.

The passed original environ data of the process is not touched
and preserved across re-execution, to allow external reading of
/proc/self/environ for process properties like container*=.

src/shared/strv.c

@@ -387,6 +387,21 @@ fail:
         return NULL;
 }
 
+char **strv_appendf(char **l, const char *format, ...) {
+        va_list ap;
+        _cleanup_free_ char *s = NULL;
+        int r;
+
+        va_start(ap, format);
+        r = vasprintf(&s, format, ap);
+        va_end(ap);
+
+        if (r < 0)
+                return NULL;
+
+        return strv_append(l, s);
+}
+
 int strv_push(char ***l, char *value) {
         char **c;
         unsigned n;

src/shared/strv.h

@@ -42,6 +42,7 @@ unsigned strv_length(char * const *l) _pure_;
 char **strv_merge(char **a, char **b);
 char **strv_merge_concat(char **a, char **b, const char *suffix);
 char **strv_append(char **l, const char *s);
+char **strv_appendf(char **l, const char *format, ...) _printf_attr_(2, 3);
 int strv_extend(char ***l, const char *value);
 int strv_push(char ***l, char *value);