Allow pg_basebackup to stream transaction log in tar mode

This will write the received transaction log into a file called
pg_wal.tar(.gz) next to the other tarfiles instead of writing it to
base.tar. When using fetch mode, the transaction log is still written to
base.tar like before, and when used against a pre-10 server, the file
is named pg_xlog.tar.

To do this, implement a new concept of a "walmethod", which is
responsible for writing the WAL. Two implementations exist, one that
writes to a plain directory (which is also used by pg_receivexlog) and
one that writes to a tar file with optional compression.

Reviewed by Michael Paquier

src/include/pgtar.h

@@ -22,4 +22,5 @@ enum tarError
 extern enum tarError tarCreateHeader(char *h, const char *filename, const char *linktarget,
 			  pgoff_t size, mode_t mode, uid_t uid, gid_t gid, time_t mtime);
 extern uint64 read_tar_number(const char *s, int len);
+extern void print_tar_number(char *s, int len, uint64 val);
 extern int	tarChecksum(char *header);