several files for signing and verification.
* rsa-sign.c, rsa-sha1-verify.c, rsa-sha1-sign.c,
rsa-md5-verify.c, rsa-md5-sign.c:  New files.

* rsa-sha1-verify.c (rsa_sha1_verify_digest): New function.
* rsa-sha1-sign.c (rsa_sha1_sign_digest):  New function.
* rsa-md5-verify.c (rsa_md5_verify_digest):  New function.
* rsa-md5-sign.c (rsa_md5_sign_digest):  New function.
* rsa-verify.c (_rsa_verify): New file, new function.

* rsa.c (_rsa_check_size): Renamed from rsa_check_size, and made
non-static. Private key functions moved to rsa-sign.c.

Rev: src/nettle/rsa-md5-sign.c:1.1
Rev: src/nettle/rsa-md5-verify.c:1.1
Rev: src/nettle/rsa-sha1-sign.c:1.1
Rev: src/nettle/rsa-sha1-verify.c:1.1
Rev: src/nettle/rsa-sign.c:1.1
Rev: src/nettle/rsa-verify.c:1.1
Rev: src/nettle/rsa.c:1.10
Rev: src/nettle/rsa.h:1.18