man filesystems: fix xrefs after move to section 4

Reviewed by: des, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1077

open.2: remove self-reference

Change .Xr reference to .Fn, which quiets a mandoc warning.

Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1135

libsys: relocate implementations and manpages

Remove core system call implementations and documentation to lib/libsys
and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>.
Update paths to allow libc to find them in their new home.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908