Revert "support for kfreebsd and old FreeBSD systems."

This reverts commit 0a214bdbb24f18c62f44f97b3ee3cb9bce15daf6.

Apparently, building nocache on kfreebsd still fails, and anyway the linker
warns that posix_fadvise will always fail. So so wait until kfreebsd is
up-to-date with the POSIX standard. (See issue #12.)
Mention Debian pkg and kfreebsd warning in README

(See Issue #12.)