Rename l to T, add cfg.user and 'TT' chat prompt

* Add cfg.user variable
* Implement TT chat prompt
* Invoked from 'VT'
* Rename 'l' to 'T' command (log -> textlog)
* Add r_sys_whoami () -> set default username
* Add $$ trap instruction in ESIL
* Implement POP and STACK esil commands
Now TT in sandbox mode and fix segfault in free