taint: enable precise_pc, it's now needed by before_block_exec
linux_vmi: move DroidScope code from core to new linux_vmi plugin

Move the Linux layer introspection API from /introspection to the linux_vmi
plugin. This means vl.c no longer has to intialize DroidScope, since the
plugin's init() function does that now.
Summary of changes:
 - configure make subdirs of {target}/panda_plugins/ for DS code, instead of
 subdirs of {target}/introspection
 - make clean wipes the files from those subdirectories. It doesn't do a
 recursive rm because the configure script makes the directories, and the
 makefile does not. Further, it wipes those specific subdirectories, and now
 deletes the pattern *.* in each directory, rather than *.
 - linux_vmi exports an API using PPP, and the fdtracker imports that API
 - DroidScope code is no longer built by Makefile.target
 - Some unused DroidScope code has been deleted instead of moved
 - Variables holding a pgd are now of type target_asid_t, which is a
 target_ulong

Note: at this time, Dalvik introspection code is included in the plugin
directory, but is not used to build the plugin.