tail: prettyname cleanup

tail: prettyname cleanup

* src/tail.c: Use ‘prettyname’ consistently as the identifier
for a prettified file name, as opposed to ‘pretty_filename’,
‘pretty_name’, and ‘name’.  This makes the code easier to follow.
(struct File_spec): New member prettyname.
(pretty_name): Remove.
All uses of pretty_name (f) replaced by f->prettyname.
(close_fd, fremote): Accept struct File_spec, not name.
All callers changed.
(main): Initialize the new prettyname member.
This is simpler/smaller than calling pretty_name everywhere.