Bug 3784: Fix crash in THREAD REFERENCES

For a particular corner case involving messages which reference
themselves and a common missing ancestor.

Based on a patch by Wolfgang Breyha.
upgrade_index: clearer error handling on missing cache/index files
Initial CalDAV code, after months of working offline.

Needs documentation badly
Initial CalDAV code, after months of working offline.

Needs documentation badly
httpd: backport to use old APIs
Added some minimal CalDAV documentation
Added HTTP compiled code
Added HTTP/WebDAV/CalDAV/iCalendar specs
include errstr in transaction context rather than passing as a separate
argument to method processing functions
Remove redundant errstr parameter from do_proppatch().
We already have an errstr in proppatch_ctx
Simplify property namespace handling for responses.
Use namespaces as given in the request body, but index them for our own
use
Fix DAV:resourcetype responses
- Fix parsing of "*" and "/" URIs (for which methods are they legal, and
  which features are allowed on them)
- Method callback functions now check ALLOW_* flags before processing
httpd: Added COPY/MOVE methods
httpd:  Remove a TODO (we are already using UNIX namespace)
httpd: Fix logging level for errors
httpd:  Updated TODO list (with some random thoughts)
httpd: Use struct buf for serverinfo string
httpd: remove check of first letter of method
httpd:  fix some nits regarding method array and unknown methods