Interface change, let all rsa signature functions have a return value.

Rev: nettle/ChangeLog:1.54
Rev: nettle/NEWS:1.4
Rev: nettle/examples/rsa-sign.c:1.2
Rev: nettle/pgp-encode.c:1.3
Rev: nettle/pkcs1-rsa-md5.c:1.3
Rev: nettle/pkcs1-rsa-sha1.c:1.3
Rev: nettle/pkcs1-rsa-sha256.c:1.3
Rev: nettle/pkcs1-rsa-sha512.c:1.2
Rev: nettle/pkcs1.c:1.3
Rev: nettle/pkcs1.h:1.3
Rev: nettle/rsa-compat.c:1.3
Rev: nettle/rsa-md5-sign.c:1.3
Rev: nettle/rsa-md5-verify.c:1.3
Rev: nettle/rsa-sha1-sign.c:1.3
Rev: nettle/rsa-sha1-verify.c:1.3
Rev: nettle/rsa-sha256-sign.c:1.3
Rev: nettle/rsa-sha256-verify.c:1.3
Rev: nettle/rsa-sha512-sign.c:1.2
Rev: nettle/rsa-sha512-verify.c:1.2
Rev: nettle/rsa.h:1.4
Rev: nettle/testsuite/cxx-test.cxx:1.3
Rev: nettle/testsuite/pkcs1-test.c:1.3
Rev: nettle/testsuite/testutils.c:1.5