MYRA RAT memfd_create fileless execution masquerading as systemd-userdbd

Detects the in-memory execution of the MYRA RAT on Linux systems. The rule identifies suspicious process activity where a process masquerades as the legitimate 'systemd-userdbd' service, while utilizing memfd_create or related file descriptor paths (e.g., /proc/self/fd/ or /memfd:) to load and execute malicious code directly from memory, thereby avoiding traditional file-based detection.