Remove duplicate ISSERVER definition
Remove off_t from what will eventually be a public API

Since the size of off_t may vary depending on whether the LFS defines
have been set, it is not a good idea to expose that, as the recently
fixed bug has shown. As long as we use nbdsrv.h internally only this
doesn't matter, but since the long-term goal is to make libnbdsrv a
public library, we'd better avoid such an issue.

We'll still use off_t internally, but that won't be exposed anymore.
Make the implementation of size_autodetect follow the documentation

The documentation claimed that size_autodetect could return UINT64_MAX
if size autodetection would fail. In fact, however, it would call err(),
which is for fatal errors.

Additionally, the final test (to use lseek SEEK_END) would never be
called, since it was preceded by an if structure which would call return
in one of its branches and err() in the other.