core: Introduce PrivateBPF= to mount a private BPFFS

Add a new option PrivateBPF= to mount a new instance of bpffs within a
namespace.
PrivateBPF= can be set to "no" to use the host bpffs in readonly mode
and "yes" to do a new mount.
The mount is done with the new fsopen()/fsmount() API because in future
we'll hook some commands between the two calls.

src/shared/bus-unit-util.c

@@ -2425,6 +2425,7 @@ static const BusProperty execute_properties[] = {
         { "MountImagePolicy",                      bus_append_string                             },
         { "ExtensionImagePolicy",                  bus_append_string                             },
         { "PrivatePIDs",                           bus_append_string                             },
+        { "PrivateBPF",                            bus_append_string                             },
         { "IgnoreSIGPIPE",                         bus_append_parse_boolean                      },
         { "TTYVHangup",                            bus_append_parse_boolean                      },
         { "TTYReset",                              bus_append_parse_boolean                      },