re PR libfortran/15235 (libgfortran doesn't build on Solaris 10)

PR fortran/15235
* gfortran.h (offset_t): Rename to ...
(gfc_offset): ... this.
* io/backspace.c (formatted_backspace, unformatted_backspace),
io/io.h (stream, gfc_unit, global_t, file_length, file_position),
transfer.c (us_read, us_write, next_record_r, next_record_w),
io/unit.c (init_units), unix.c (unix_stream, fd_alloc,
fd_alloc_r_at, fd_alloc_w_at, fd_seek, mmap_alloc,
mmap_alloc_r_at, mmap_alloc_w_at, mmap_seek, mem_alloc_r_at,
mem_alloc_w_at, mem_seek, file_length, file_position): Replace all
occurences of offset_t by gfc_offset.

From-SVN: r81994