bsd-user: spelling fixes: necesary, agrument, undocummented

Fixes: a99d7403 "bsd-user: Implement do_obreak function"
Fixes: 86327290 "bsd-user: Implement freebsd_exec_common, used in implementing execve/fexecve."
Fixes: bf14f13d "bsd-user: Implement stat related syscalls"
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>