fseek to end of known_hosts before writing to it. POSIX and ANSI C

fseek to end of known_hosts before writing to it.  POSIX and ANSI C
require that applications call fseek or similar between read and writing
to a RW file.  OpenBSD doesn't enforce this, but some (System V derived)
platforms need this to prevent it from writing a spurious extra byte (in
this case, a newline).  ok djm@ deraadt@