rework pam, ncurses and termcap include files

This allows error checking inside the included files instead of either
including the appropriate file or a blank file.

Also the blank file named .mk gets removed by this change.

mk/termcap.mk

@@ -1,3 +1,11 @@
+ifeq (${MKTERMCAP},ncurses)
+LIBTERMCAP?=	-lncurses
+CPPFLAGS+=	-DHAVE_TERMCAP
+LDADD+=		${LIBTERMCAP}
+else ifeq (${MKTERMCAP},termcap)
 LIBTERMCAP?=	-ltermcap
 CPPFLAGS+=	-DHAVE_TERMCAP
 LDADD+=		${LIBTERMCAP}
+else ifneq (${MKTERMCAP},)
+$(error If MKTERMCAP is defined, it must be ncurses or termcap)
+endif