resip/recon: allow multiple values for each extension header

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@11061 ddefafc4-47db-0310-ae44-fa13212b10f2
resip/stack: DTMF: validate key names in dtmf-relay body

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@11062 ddefafc4-47db-0310-ae44-fa13212b10f2