Genie:  Add and use file mark accessors

Rename Advance() and GetFileMark() to advance_mark() and get_mark(), and
add set_mark().  Use them instead of accessing itsMark directly.
vfs, Genie:  Move file mark member to filehandle
vfs, Genie:  Rewrite Seek() member as vfs function
Merge branch 'seek'
vfs, Genie:  Remove IsRegularFile() member