cli: use sig_atomic_t type for variable modified by SIGINT handler

Other nits:
* removed unused b_exit_on_ctrl_c
* removed unnecessary static initializer
do not check for NULL prior to delete, do not reset pointers in destructors

A lot of our classes have destroy() methods. In those we try to zero deleted
pointers since there is a chance for multiple destroy() calls. But in
destructors the pointer storage itself will be released when the function exits
TEncTop: replace debugging exit() call with abort(), use x265_log
TEncSbac: remove exit() calls from unsupported functions
cli: report errors from registering signal handler