zsh_reload: use `exec zsh` instead of `source ~/.zshrc`

`source ~/.zshrc` is a bad practice, it doesn't make sure that the full
zsh session is reloaded and it may have side effects.

Use `$SHELL` as the path to zsh if it exists (this will fix edge cases
where the zsh used is not the first on $PATH). Otherwise, use `zsh`.