Added ssize_t to configure code.

Added ssize_t to configure code.
Got 'religion' about using size_t and ssize_t for read/write stuff
as part of the code to expose 64 bits to the client.

This checkin does all the 'easy' stuff - such as all the read/write/lock
calls - but now comes the harder parts (open & friends) and all the
file enquiry functions.....

Jeremy.
(This used to be commit 36544fe5476f7770bd5748574fc54be7b3ee4d4a)