LinuxMonitor: add support for signals

[vitalych]

Currently, one needs to manually register a signal handler in the guest program to catch them and notify S2E. An alternative would be to instrument the kernel, like what is already done for segfaults and traps.

Check the last commits in the s2e-linux-kernel repo to see an example for segfaults and traps. Implementing signals is very similar. The kernel would essentially send a message to LinuxMonitor when there is a signal.