Only set CURLOPT_COOKIEFILE, if it exists

[blueyed]

It looks like setting CURLOPT_COOKIEFILE in createEasy (web.c) is causing regular disk access (and therefore preventing the disks to shutdown).

I am seeing entries like the following when enabling the hibernate debug tool with a Synology Diskstation (syno_hibernate_debug_tool --enable 1):

kernel: [15382.650000] [/usr/local/transmission/var//cookies.txt] opened by pid 6752 [u:(/usr/local/transmission/bin/tra), comm:(transmission-da)]

As far as I can see, the COOKIEFILE is only used to read cookies from. Therefore it might be acceptable to require a restart of transmission in case you manually edit it?!

This way, transmission could check if the file exists during startup and only set the option in case a cookie file existed during startup.

There _might_ be a way to make the createEasy function check for the file's existence without actually hitting the disk (I could imagine that there are caches/methods which actually remember if a file has been created?!).

libcurl 7.24.0-1 is being installed on the system (via ipkg), but I am not sure, if there is another instance used by transmission?!

The strace output for the access is:

[pid  X] open("/usr/local/transmission/var//cookies.txt", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)

[jordan]

This is a good idea.

[jordan]

Fixed in r14075, r14076