Fix M-x eglot prompt when connection already exists (bug#60557)

Fix M-x eglot prompt when connection already exists (bug#60557)

Before this change, if a current connection existed at the time of M-x
eglot, user would be first asked to enter into M-x eglot's interactive
spec details about new command line arguments, and only afterwards be
prompted about what to do with the current connection (which could be
to reconnect to it using the same arguments, or tear it down and make
a new one).

This is very confusing, as users may not be fully aware of the
distinction between "reconnect" vs "disconnect-and-connect".  They
might not know if any new command line arguments provided are taking
effect or not.

This change simplifies this and removes the option to "reconnect
instead" from M-x eglot (users can do that at any time via M-x
eglot-reconnect).  It also ensures that users are informed about a
current connection before asking to enter new command line arguments
and not the other way round.

* lisp/progmodes/eglot.el (eglot): Rework.