Fix semantics of -K used together with explicit member names.

Fix semantics of -K used together with explicit member names.

This also fixes the bug reported in
  http://lists.gnu.org/archive/html/bug-tar/2018-12/msg00012.html

* src/common.h (starting_file_option): Describe the variable.
* src/names.c (add_starting_file): New function.
(name_match): Ignore everything before the member indicated by the
--starting-file option
* src/tar.c: Use add_starting_file to handle the -K option.