Add basic write support for 'pax' format archives

Add basic write support for 'pax' format archives

Keep writing archives in ustar format by default.  People can test the
posix 'pax' format using pax(1) -w -x pax ... or cpio -o -H pax ...;
tar(1) can't exercise this code yet.  Only long names file and link
names are supported for now.

With input and tests from caspar@, ok millert@