terminal-util: use the right ANSI ST sequence

terminal-util: use the right ANSI ST sequence

There are multiple ways to encode ANSI ST, and we generally prefer ESC \
for it, for reasons explained in terminal-util.h. Hence, let's actually
follow this rule in the terminal reset logic, and use the ANSI_ST macro.

This will change the byte sequence generated (as it means we use ESC \
rather than BEL), but it doesn't change behaviour, as the two sequences
should be equivalent.