LegacyBinary- support for legacy binary codes

LegacyBinary: support for legacy binaries in PadicoTM.

Rationale:

.1. padico-launch sets attribute PADICO_LEGACY=<binary>

.2. padico-d sets a LD_PRELOAD=padico-legacy.so

.3. library static initializer from padico-legacy.so bootstraps Puk and PadicoTM

.4. the session script loads & starts a 'legacy' module

.5. unit_legacy_start() (from this file) is called and starts a thread

.7. the new thread calls padico_legacy_start() (from padico-legacy.c)

.8. padico_legacy_start() calls the real main from the application.