Replace all tests that require knowing syscall numbers.

Replace all tests that require knowing syscall numbers.

Since tests that need syscall.pm or sys/syscall.pm, et al., always
fail on the CPAN testing systems for one of several reasons (causing
"make test" to fail because loopback.pl can't even create plain
files without it), I'm adopting the *BSD arrangement for handling
mknod() in loopback.pl, and using the Lchown module to do lchown()
instead of trying to lookup a syscall number for it. Also changed
the statfs test to do the same, as I'd already written code to
use Filesys::Statvfs instead of trying to track down the
statfs/statvfs/statvfs1 syscall number, and worry about packing
the arguments the right way. Also changed Makefile.PL to provide
per-platform explanations of what to install for FUSE support, and
moved the "-g -ggdb" options into the OPTIMIZE parameter to
WriteMakefile(). Also made a note of testing against CentOS 5.6,
due to its use of Perl 5.8, as opposed to everything else I'd
tested against prior.