New function ecc_mul_m.

* curve25519-mul.c (curve25519_mul): Use ecc_mul_m.
* curve448-mul.c (curve448_mul): Likewise.

* ecc-mul-m.c (ecc_mul_m): New file and function. Implements
multipliction for curves in Montgomery representation, as used for
curve25519 and curve448. Extracted from curve25519_mul.
* ecc-internal.h (ecc_mul_m): Declare.
(ECC_MUL_M_ITCH): New macro.
* Makefile.in (hogweed_SOURCES): Add ecc-mul-m.c.