shutdown pluto using various magic numbers

The --shutdown and --status commands set .magic to WHACK_BASIC_MAGIC.
This way they work regardless of the libreswan version.

For instance:

- after an upgrade, a 5.2 whack can shutdown the running 5.1 pluto
- after a downgrade, a 5.1 whack can shutdown the running 5.2 pluto

Prior to 5.1 this was all broken:

- .magic was always WHACK_MAGIC
- WHACK_MAGIC wasn't updated
- .whack_shutdown kept moving

consequently .whack_shutdown's location was only correct when whack
and pluto matched.  For instance, a v4.x <<whack --shutdown>> would be
interpreted by v5.0 as <<whack --showstates>> (because string encoding
changed this would result in a core dump).

See github/1709

Note: init systems should use SIGTERM to shutdown pluto.
