Reduce issues with types by switching iosz and totalsz from off_t to size_t.

Reduce issues with types by switching iosz and totalsz from off_t to size_t.
In rpki-client the maximum file size is limited to 2GB so even on 32bit
archs size_t is large enough.
This solves some of the signed vs unsigned issues between bufsz/bufpos
and iosz.
OK tb@