This patch implements the POSIX signal functionality needed to interrupt a
running Noux GDB by pressing 'Ctrl-C'.
It allows to register a signal handler for the 'SIGINT' signal, which
gets executed after 'Ctrl-C' is received from the terminal. With the
current state of the implementation, the signal handler only gets executed
when the Noux application calls a 'read()', 'write()', 'ftruncate()' or
'select()' syscall.
Fixes#923.
