file_seek() used to be a wrapper around fseek() or gzseek(), both of

file_seek() used to be a wrapper around fseek() or gzseek(), both of
which could use lseek() and were thus expensive due to system call
overhead.  To avoid making a system call for every packet on a
sequential read, we maintained a data_offset field in the wtap structure
for sequential reads.

It's now a routine that just returns information from the FILE_T data
structure, so it's cheap.  Use it, rather than maintaining the data_offset
field.

Readers for some file formats need to maintain file offset themselves;
have them do so in their private data structures.

svn path=/trunk/; revision=42423