ike-sa: Add possibility to store private extensions/conditions

ike-sa: Add possibility to store private extensions/conditions

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)
	{
	}