This avoids conflicts with upstream changes if patched versions of
strongSwan require a number of private extensions and conditions. For
example, the following extensions can be used as usual via the
`enable|supports_extension()` methods:
#define PRIVATE_EXT_1 (EXT_PRIVATE_MARKER | (1<<0))
#define PRIVATE_EXT_2 (EXT_PRIVATE_MARKER | (1<<1))
Defining an enum would also be possible but because the type won't match
the values would have to be cast to `ike_extension_t` when using the
methods.
Similarly, `COND_PRIVATE_MARKER` may be used to define private conditions
that can be used with the `set|has_condition()` methods.
Because the MSB is explicitly not set in `private_extensions|conditions`,
these members may directly be checked against private values, e.g.:
if (this->private_extensions & PRIVATE_EXT_1)
{
}
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