tar: remove unused externs, or make them static

* src/buffer.c (record_buffer_aligned, flush_write_ptr, flush_read_ptr)
(duration):
* src/list.c (recent_global_header):
* src/unlink.c (deferred_unlink_delay):
* src/transform.c (transform_flags):
* src/xattrs.c (acl_get_file_at, acl_set_file_at, file_has_acl_at):
Now static.
* src/common.h (rmt_command_option): Remove; unused.
* src/tar.c (check_links_option, allocated_archive_names):
Move decls here from common.h; these are now static.