make(1): start replacing Lst_Find with Lst_FindB

Lst_Find is called with a "comparison" function that returns the integer
0 if the desired node is found.  This leads to confusion since there are
so many different return value conventions for int, such as 0/1 for
mimicking false/true, -1/0 as in close(2), and the sign as in strcmp(3).
This API is much easier to understand if the "comparison" function is
not called a comparison function (since that is too close to strcmp),
but a "match" function that just returns a boolean.

In Lst_FindFromB, the node argument may be null.  This deviates from the
other Lst functions, which require Lst and LstNode to generally be
non-null.  In this case it is useful though to make the calling code
simpler.

In arch.c, this makes a lot of the previous documentation redundant.

In cond.c, the documentation is reduced a little bit since it had
already been cleaned up before.  It also removes the strange negation
from CondFindStrMatch.

In dir.c, the documentation collapses as well.

In main.c, separating the ReadMakefile function from the callbacks for
Lst_FindB allows the former to get back its natural function signature,
with proper types and no unused parameters.

To catch any accidental mistakes during the migration from Lst_Find to
Lst_FindB, the code can be compiled with -DUSE_DOUBLE_BOOLEAN, which
will complain about incompatible function pointer types.

(rillig)