Make tparam.h and terminfo.c consistent.

Make tparam.h and terminfo.c consistent.

* cm.c (tputs, tgoto, BC, UP): Remove extern decls.  Include
tparam.h instead, since it declares them.
* cm.h (PC): Remove extern decl; tparam.h now does this.
* deps.mk (cm.o, terminfo.o): Depend on tparam.h.
* terminfo.c: Include tparam.h, to check interfaces.
(tparm): Make 1st arg a const pointer in decl.  Put it at top level.
(tparam): Adjust signature to match interface in tparam.h;
this removes some undefined behavior.  Check that outstring and len
are zero, which they always are with Emacs.
* tparam.h (PC, BC, UP): New extern decls.