POSIX extended format headers do not include PID by default

The intent is to make binary-equivalent PAX archives easy to create.  If
POSIXLY_CORRECT is set, the POSIX standard default is used, which embeds
the pid.

* src/common.h (posixly_correct): New global.
* src/tar.c (decode_options): Detect the POSIXLY_CORRECT environment
variable.
* src/buffer.c (add_chunk_header): Change filenames of multipart files to
omit the pid.
* src/xheader.c (HEADER_TEMPLATE): New macro.
(xheader_xhdr_name, xheader_ghdr_name): Use HEADER_TEMPLATE to select
the template for the POSIX extended header name.
* doc/tar.texi: Document the change.

Signed-off-by: Zachary Vance <za3k@za3k.com>

src/common.h

@@ -340,6 +340,9 @@ GLOBAL const char *volume_label_option;
 
 /* Other global variables.  */
 
+/* Force POSIX-compliance */
+GLOBAL bool posixly_correct;
+
 /* File descriptor for archive file.  */
 GLOBAL int archive;