TLS implementation

Jerry,

I have a number of questions about how the TLS implementation
interacts with the build system and to some extent the Lisp engine.  I
know you're very busy, so if you don't have time to do any review or
answer my question, I'd appreciate a note saying so and I'll just
proceed with caution.

(0) I think we ought to default this feature on.  I fuzzily remember
    that I wanted some testing before doing that, but most of the
    build issues should be worked out by now, and the "fill can
    relocate strings" issue has been fixed with Aidan's implementation
    of clear_passwd.

(1) I guess the reason for always building and linking tls.c is to
    define 'tls-error.  Is that correct?

(2) If I configure and build --without-tls, I get a warning because
    Qread_passwd is an unused variable.  I don't see any reason not to
    DEFSYMBOL it unconditionally, it's just one slot in the obarray
    and this is a pretty important function.

(3) I'm very tempted to move the definitions of 'tls-error to
    process.c (and maybe make it a sub-error of 'network-error) and
    'read-password to general-slots.h.  Any reason not to do that?

(4) Once (3) is done, I believe that tls.c doesn't need to be compiled
    or linked if configured --without-tls.  Any reason not to do the
    usual Makefile dance for conditionally compiled modules that you
    know of?

Regards,

Steve