tar: extract permissions for . last

* src/common.h (must_be_dot_or_slash): New decl.
* src/extract.c (mark_after_links): New function, taking code
that used to be in create_placeholder_file.
(create_placeholder_file): Use it.
(delay_set_stat): Always delay setting status for . and /.
* src/misc.c (must_be_dot_or_slash): Now extern.
* tests/extrac12.at: New file.
* tests/Makefile.am (TESTSUITE_AT): Add it.
* tests/testsuite.at: Likewise.