Fix authentication with FreeRDP 3.32

FreeRDP 3.32 enables FreeRDP_ExtSecurity by default. KRDP explicitly selects TLS or NLA depending on the authentication mode, but does not currently configure extended security.

With FreeRDP 3.32.1, this allows PROTOCOL_HYBRID_EX to be selected for PAM/system-user authentication. Authentication then enters CredSSP/NTLM before KRDP's PAM authentication and fails because there is no corresponding user in the SAM database.

The resulting errors include:

[ERROR][com.winpr.sspi.NTLM] - [ntlm_fetch_ntlm_v2_hash]: Error: Could not find user in SAM database
[WARN][com.winpr.sspi] - [winpr_AcceptSecurityContext]: AcceptSecurityContext status SEC_E_NO_CREDENTIALS [0x8009030e]
[ERROR][com.freerdp.core.auth] - [credssp_auth_authenticate]: AcceptSecurityContext status SEC_E_NO_CREDENTIALS [0x8009030e]
[ERROR][com.freerdp.core.transport] - [transport_accept_nla]: client authentication failure

Disable FreeRDP_ExtSecurity for PAM authentication so that KRDP's existing TLS/PAM authentication path is used, while leaving extended security enabled for the SAM-backed NLA path used by KRDP-managed users.

Reproduction

Tested with KRDP 6.7.5 and FreeRDP 3.32.1:

• A normal Windows Remote Desktop (MSTSC) connection using system-user authentication fails with the errors above when FreeRDP_ExtSecurity is left at its new default.
• A FreeRDP client forced to /sec:tls successfully authenticates the same system user.
• With FreeRDP_ExtSecurity disabled for PAM/system-user authentication, an ordinary Windows Remote Desktop client successfully authenticates the system user.
• With FreeRDP_ExtSecurity enabled for KRDP-managed/SAM authentication, an ordinary Windows Remote Desktop client successfully authenticates a KRDP-managed user.
• Incorrect passwords are rejected for both the PAM/system-user and KRDP-managed/SAM authentication paths.

Current master still leaves FreeRDP_ExtSecurity unspecified. I was unable to runtime-test current master on the same system because it requires newer KPipeWire APIs than Plasma 6.7.5 provides. The runtime authentication tests were performed against KRDP 6.7.5.