This is part of #353. This commit prepares for the transition to our own
Signal type by defining the type and implementing some basic operations.

This commit adds a new variant Signal::Number(c_int) to allow specifying
any signal by its raw signal number. This will make the parser for the
trap built-in condition operand much simpler.

This commit replaces the SigSet type with an array of Signal in the
System trait. This change is necessary because the SigSet type is
specific to the nix crate, and the System trait should not depend on
specific types from external crates.

This commit also updates the implementations of the System trait to
reflect this change. SelectSystem::wait_mask is now an optional Vec of
Signal, and virtual Process now uses a BTreeSet of Signal instead of
SigSet.

This change makes `rt_range` return an empty range when the real system
does not support real-time signals. This simplifies the code that uses
`rt_range` because the caller does not need to handle the `None` case.

The conversion from ProcessState to ExitStatus is now done by the
System trait as it depends on the system implementation. This commit
removes the TryFrom implementation and replaces it with a call to
System::exit_status_for_process_state.

The conversion should be done by `SystemEx::exit_status_for_signal`.