linker: Make linker.h more self-contained

struct kld_file_stat embeds a reference to MAXPATHLEN, defined in
param.h.

PR: 280432
MFC after: 2 weeks

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

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

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