fpaste: add register command for public certificate enrollment

- Add register command to obtain client cert from server
- Solve PoW challenge, receive PKCS#12 bundle
- Extract cert/key, optionally update config (--configure)
- Fix registration to work without PKI_ENABLED (only needs PKI_CA_PASSWORD)
- Add skip_enabled_check param to get_ca_info() for registration path
- Update docs: README examples, API header name fix (X-Fingerprint-SHA1)

README.md

@@ -143,6 +143,17 @@ echo "Hello" | ./fpaste
 
 # Show server info
 ./fpaste info
+
+# Register and get client certificate (if server supports it)
+./fpaste register
+# Saves certificate to ~/.config/fpaste/
+
+# Register with auto-configuration
+./fpaste register --configure
+# Creates cert files and updates config with fingerprint
+
+# Register with custom name
+./fpaste register -n my-laptop --configure
 ```
 
 ### End-to-End Encryption