Cleanup IKEv2 code so it builds again
multiple fixes and minor additions
* fixed json-c header inclusion for new upstream install location
* fixed server string building
* added json-c library to version checking
Add support for searching in 'prefix' dirs for headers
Don't use prefix variable (used by AC)
Disable NULL context tracking
Alloc handlers from the NULL context
Rbtree shouldn't be allocated from inst
Alloc connections pools in the NULL ctx
Don't parent the rlm_cache rbtree directly from inst
Same fix for eap session and handler trees