pjsip_msg_find_hdr(), pjsip_msg_find_hdr_by_name(), and
pjsip_msg_find_hdr_by_names() require a pjsip_msg to be passed in
so if you need to search a header list that's not in a pjsip_msg,
you have to do it yourself.  This commit adds generic versions of
those 3 functions that take in the actual header list head instead
of a pjsip_msg so if you need to search a list of headers in
something like a pjsip_multipart_part, you can do so easily.

Change-Id: I6f2c127170eafda48e5e0d5d4d187bcd52b4df07