Doc: Update manpages for KEYCTL_DH_COMPUTE

Created a new manual page for section 3 for the keyctl_dh_compute and
keyctl_dh_compute_alloc functions and listed these functions in the
man3 page for keyctl. Updated the man1 page for the new 'keyctl
dh_compute' command.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>