tree-wide: remove support for kernels lacking ambient caps

Let's bump the kernel baseline a bit to 4.3 and thus require ambient
caps.

This allows us to remove support for a variety of special casing, most
importantly the ExecStart=!! hack.

src/shared/exec-util.h

@@ -48,9 +48,9 @@ typedef enum ExecCommandFlags {
         EXEC_COMMAND_IGNORE_FAILURE   = 1 << 0,
         EXEC_COMMAND_FULLY_PRIVILEGED = 1 << 1,
         EXEC_COMMAND_NO_SETUID        = 1 << 2,
-        EXEC_COMMAND_AMBIENT_MAGIC    = 1 << 3,
-        EXEC_COMMAND_NO_ENV_EXPAND    = 1 << 4,
+        EXEC_COMMAND_NO_ENV_EXPAND    = 1 << 3,
         _EXEC_COMMAND_FLAGS_INVALID   = -EINVAL,
+        _EXEC_COMMAND_FLAGS_ALL       = (1 << 4) -1,
 } ExecCommandFlags;
 
 int exec_command_flags_from_strv(char * const *ex_opts, ExecCommandFlags *ret);