make(1): rename confusing function ReadAllMakefiles

The old name implied that the function would read multiple files, which
was not the case.

The comment above that function was highly confusing.  It's not that the
function returns a boolean, but rather 0 or non-zero, and 0 means that
Lst_Find should stop searching.

One of the next refactorings will be to make Lst_Find return the first
list node for which the function returns TRUE.  This will reduce the
confusion about the several functions called SomethingP in suff.c.  The
P suffix means to return TRUE or FALSE, not 0 or non-zero.

(rillig)