The tar/archive code from golang has a problem with linknames with length >
100. A pax header is added but the original header still written with a too
long field length.

As it is clear that pax support is incomplete I have added missing
implementation parts.

This commit contains code from the golang project in the folder tar/archiv.

The following pax header records are now automatically written:

- gname)
- linkpath
- path
- uname

The following fields can be written with PAX, but the default is to use the
star binary extension.

- gid  (value > 2097151)
- size (value > 8589934591)
- uid (value > 2097151)

The string fields are written when the value is longer as the field or if the
string contains a char that is not encodable as 7-bit ASCII value.

The change was tested against a current ubuntu-cloud image tarball comparing
the compressed result.

+ added some automated tests for the new functionality.

Fixes #6056.

R=dsymonds
CC=golang-dev
https://golang.org/cl/12561043