Attached patch adds the zrestart() function that was discussed
previously in workers/48408 and before (and also after).

It was suggested originally that `zle -fn` be used to validate the
dotfiles. However, I discovered that this can fail needlessly, for
example, if a dotfile references dynamically named directories.
Instead, to verify that the shell can restart without errors, I start
an interactive subshell with an empty command and capture its return
value and standard error.

Likewise, it was suggested at some point that some form of `zsh -l &&
exit` be used instead of `exec zsh`. However, it was later brought up
that, if the user would restart the shell many times this way, this
could potentially exhaust the available process IDs. Thus, I use `exec
zsh` after all. Hopefully, the validation above provides enough
safeguards. At least the user doesn't have to worry about mistyping
`zsh` as the argument to `exec` this way.