books/bookvol5, books/bookvol9, src/interp/* functionp -> canFuncall?

Axiom has a function, originally in BOOT, called functionp.
BOOT is case sensitive so this function is called |functionp| in lisp.
Lisp uppercases symbols so functionp becomes FUNCTIONP.

This led to a subtle failing change, found by Camm Maguire.
The BOOT-derived function is now called canFuncall?