chain.c, etc.: Add POSIX hsearch(3) and tsearch(3) support.

chain.c, etc.: Add POSIX hsearch(3) and tsearch(3) support.

GNU libc's hsearch(3) is surprisingly fast on its own, but it's sadly
fixed size, and there's no fast way to resize it.  So there are two
implementations, one of which will grow the table on demand, and the
other just allocates it big enough to begin with.

(Actually, they're the same, but the `fixed size' version just makes a
huge initial allocation.)