MFC r217064 - sort & fix includes, remove -include from CFLAGS of acct tests

MFC r217064 - sort & fix includes, remove -include from CFLAGS of acct tests

- Sort the includes of pack.c, moving sys/*.h files near the top.
- Add a couple of missing #include lines, and remove the need for
  custom -include options in the CFLAGS of the test Makefile.
- Remove the ad-hoc ’all’ target, but keep its ’regress’ bits for
  testing.
- Convert the ad-hoc ’clean’ target to proper CLEANFILES stuff,
  so that the normal bsd.prog.mk machinery can clean up.
- Use ‘make -V .OBJDIR’ to detect the place where ’pack’ lives,
  so that regress.t works both with and without ’make obj’.

git-svn-id: svn://svn.freebsd.org/base/stable/8@217599 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f