Fuzzer harnesses for sig verify and pubkey parsing

These are some basic clang libfuzzer harnesses for signature
verification and public key parsing. Some assembly (metaphorical)
required.
diff --git a/.gitignore b/.gitignore
index e25a830..650eb3c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,3 +25,4 @@
 ssh-keysign
 ssh-pkcs11-helper
 sshd
+!regress/misc/fuzz-harness/Makefile