Revert "http_dav.c: message only gets mapped for a successful GET"

This reverts commit 21a3dae0de75fd4ef0bff184f4a238720e0f67e8.

(I'm an idiot: the message needs to be mapped until we output the data)
jcal.c, configure.in: switch to using jansson rather than json-c
don't make an unnecessary copy of iCal/jCal/xCal data
http_ischedule: add support for xCal and advertise xCal and jCal
jcal.c: match jansson output when dumping entire calendar
httpd.c: add txn flag for MIME-conformant responses and use it for multipart
httpd.c: Server header should advertise libxml2, not libxml
httpd.c: added support for X-HTTP-Method-Override (not sure why, but it was trivial and secure)
http_proxy.c: don't return an error when piping a zero-length response
http_proxy.c: pipe PUT response body to client when doing multi-backed COPY
http_proxy.c: include more client request headers in PUT half of multi-backend COPY
http_proxy.c: when piping a request, forward any client-supplied TE header to backend
httpd.c, http_*dav.c: properly handle quality values (especially 0.0) in Accept header
http_*dav.c: always obey Accept header for GET and free-busy REPORT
http_*dav.c: obey Accept header for PUT + Prefer:return=representation
http_dav.c: fix broken LOCK response - missing status line and headers for 200 (OK)
http_proxy.c: add LOCKing to http_proxy_copy() when in MOVE mode
http_*dav.c: consolidate "get data" propfind functions
	     add support for content-type attribute when fetching CALDAV:calendar-timezone
http_caldav.c: add support for content-type attribute when storing CALDAV:calendar-timezone
install-http.html: updated