Opens the shared library at `path` (via dlopen/LoadLibrary) and
returns a positive library handle for use with ffi_symbol() and
ffi_prepare(). Returns 0 on failure; call ffi_error() for the reason.
An empty `path` opens the driver's own process image, giving access
to already-linked C symbols such as libc/libm (`sqrt`, `abs`, ...).
Every call is gated by the master apply valid_ffi("load", path,
caller); the default master denies it. This efun is only present when
the driver is built with package_ffi (`__PACKAGE_FFI__`).