diskd patch

Hi,

The following patch makes some of the diskd resource shortages
more logging-friendly. Specifically, storeDiskdShmGet() can
return NULL which the IO code will then use as a cue to
fail the operation (and count it). This solves the occasional
email to squid-users wondering why an assert() triggers in
storeDiskdShmGet().

Comments? I'd like to commit this reasonably soon.

Adrian