child-cfg: Optionally use 96-bit truncation for HMAC-SHA-256

The correct truncation is 128-bit but some implementations insist on
using 96-bit truncation.  With strongSwan this can be negotiated using
an algorithm identifier from a private range.  But this doesn't work
with third-party implementations.  This adds an option to use 96-bit
truncation even if the official identifier is used.

src/libcharon/config/child_cfg.h

@@ -307,6 +307,9 @@ enum child_cfg_option_t {
 
 	/** Enable hardware offload, if supported by the IPsec backend */
 	OPT_HW_OFFLOAD = (1<<5),
+
+	/** Force 96-bit truncation for SHA-256 */
+	OPT_SHA256_96 = (1<<6),
 };
 
 /**