event_loop_select: remove chatty debug messages
http_server: add various logging
http_server: communicate keep-alive time via headers
http_server: add ability to force "connection: close" semantics

http_server automatically decides on using a keep-alive connection
depending on what the client advertises. A http request handler
might want to force "connection: close" semantics on a request
for various reasons, this change allows that.a
webdav_server: always use "connection: close" semantics

The Mac OS X webdav mounter doesn't handle keep-alive well.
In particular, user programs that open() a file without
calling write() for a while will fail a subsequent write().
Arguably this is the fault of buggy user program but failing
write() is a rare occurence. To avoid a spuriously failing write()
the webdav mounter should reopen the connection when a request
failes but it doesn't. This change makes it so the webdav mounter
doesn't attempt to reuse connections in the first place.