[Fix] cryptobox: properly bypass RHEL/CentOS 10+ crypto-policies for SHA-1 DKIM

[Fix] cryptobox: properly bypass RHEL/CentOS 10+ crypto-policies for SHA-1 DKIM

RHEL/CentOS 10+ disables SHA-1 signatures via the rh-allow-sha1-signatures
config option, which is not bypassed by simply creating a new OSSL_LIB_CTX.

This fix:
- Creates a temporary OpenSSL config file with rh-allow-sha1-signatures=yes
- Loads it into the dedicated SHA-1 library context via OSSL_LIB_CTX_load_config
- Improves error messages to include algorithm name and RHEL-specific hints
- Captures OpenSSL error details when EVP_PKEY_verify fails
- Adds troubleshooting guidance in error messages

On non-RHEL systems, the config option is simply ignored.