Use file's mtime as mtime for its extended header.

This makes two pax archives binary equivalent if they
have the same contents and care is taken to make extended
headers otherwise reproducible, e.g. by using:

  --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0

Proposed by Michael D. Adams <mdmkolbe@gmail.com>.

* src/common.h (start_private_header): Take time_t as 3rd param.
(xheader_write): Likewise.
* src/create.c (start_private_header): Take time_t as 3rd param.
All callers updated.
(write_extended): Use file's mtime as mtime for its extended header,
Use current time stamp as mtime for global headers.
(xheader_write): Take time_t as 3rd param.