sandbox: skip core/file.c core/load.c on ENABLE_SANDBOX
sandbox: rename ENABLE_SANDBOX to SANDBOX
sandbox: finish layout changes

disable load and file objects, buffile lib.
disable some aio classes:
  fs, fs_event, fs_poll, process classes, kill and spawn methods.
statically linked: readline, aio.
disable -L cmdline, external compilers (load disabled)
moved io_error to internal.c
sandbox #64: Init static libs

without -DSTATIC (yeah)
add static DYNLIBS to libpotion.a
symlink bin/potion to bin/potion-s
defaults to bin/potion-s
sandbox: add instructions in INSTALL.md
doc: add external dependencies to INSTALL.md

from p2