Change name of ibcmp to foldEQ

Change name of ibcmp to foldEQ

As discussed on p5p, ibcmp has different semantics from other cmp
functions in that it is a binary instead of ternary function.  It is
less confusing then to have a name that implies true/false.

There are three functions affected: ibcmp, ibcmp_locale and ibcmp_utf8.
ibcmp is actually equivalent to foldNE, but for the same reason that things
like 'unless' and 'until' are cautioned against, I changed the functions
to foldEQ, so that the existing names, like ibcmp_utf8 are defined as
macros as being the complement of foldEQ.

This patch also changes the one file where turning ibcmp into a macro
causes problems.  It changes it to use the new name.  It also documents
for the first time ibcmp, ibcmp_locale and their new names.