Define `#system_echo` and `#system_raw` on all systems

[HertzDevil]

Follow-up to #12294. The breaking changes are:

• #noecho! and #raw! return nil instead of LibC.tcsetattr's return value;
• The *_from_tc_mode! macros are now deprecated.

Later we should probably deprecate Termios as well, since it is now only used from within src/crystal/system/unix/file_descriptor.cr.

I have no idea if these functions make sense on WASI. They are left unimplemented.