Sync with FreeBSD cp.

Sync with FreeBSD cp.

o Bump FreeBSD CVS IDs and manual page date
o Style(9)
o Constify
o Add SIGINFO handler (Author: mdodd, 1.40)
o Don't mmap(2) and munmap(2) zero-length files. (Author: alc, 1.42)
o Note how cp(1) handles directories ending in "/" (Author: trhodes, 1.29.2.1)
o Sync program's usage() with manpage's SYNOPSIS. (Author: ru, 1.45)
o Include <signal.h> instead of depending on namespace pollution in
  <sys/param.h>.  Include <sys/types.h> instead of of <sys/param.h>
  so that further such dependencies don't develop. (Author: bde, 1.47)

Submitted-by: Devon H. O'Dell <dodell@sitetronics.com> and
              Sarunas Vancevicius <svan@redbrick.dcu.ie>
Reviewed-by: joerg
Some-minor-changes: corecode