Format signature + minor fixes

* Added format signature that serves two purposes:
  * allows ptar to skip to archive beginning in eg. mail messages and
  * adds format versioning, so that future format changes could be easier to cope with.
* Made "install.sh" apply CPPFLAGS - this is needed for BSD ports and (probably) for some Linux distros too;
* "func()" declarations are suboptimal - they have undefined argument lists, which may hinder portability in future. As the code currently lacks function prototypes I changed such declarations to "func(void)".