Introduce trivial new pager.c helper infrastructure

This introduces the new function

void setup_pager(void);

to set up output to be written through a pager applocation.

All in preparation for doing the simple scripts in C.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>