signal: add track_caller to public APIs

[hds]

Motivation

When a user of tokio calls a function that panics when misused (e.g. calling
signal() without having an IO driver enabled in the surrounding tokio runtime)
then the user currently sees the line number of the panic call inside tokio. It
would be more informative for the user to see the place where they called the
panicking function.

It is still possible for the user to see the full stack trace by setting the
environment variable RUST_BACKLOG=1, so no useful information is
hidden.

This change is the 6th in a series towards closing #4413 (starting with #4772),
this change is for the signal functions in the main tokio crate.

Solution

Functions that may panic can be annotated with #[track_caller] so that
in the event of a panic, the function where the user called the
panicking function is shown instead of the file and line within Tokio
source.

This change adds #[track_caller] to the signal() function which is the
only function in the signal public API which can panic. Documentation
was added to this function to indicate that it may panic.

Not all panic cases can have #[track_caller] applied fully as the
callstack passes through a closure which isn't yet supported by the
annotation (e.g. signal() called from outside a tokio runtime).

Tests are included to cover the case where signal() is called from a
runtime without IO enabled.

Refs: #4413