services/ssh-agent: treat SIGTERM exit as clean

OpenSSH ssh-agent exits with status 2 when systemd stops it in
non-socket-activated mode.

Home Manager runs ssh-agent that way, so normal user-manager
shutdowns show up as unit failures.

Set SuccessExitStatus=2 for the Linux user service to match
upstream behavior. Startup failures and other unexpected exits
still fail the unit.

tests/modules/services/ssh-agent/linux/basic-service-expected.service

@@ -3,6 +3,7 @@ WantedBy=default.target
 
 [Service]
 ExecStart=@openssh@/bin/ssh-agent -D -a %t/ssh-agent/socket
+SuccessExitStatus=2
 
 [Unit]
 Description=SSH authentication agent

tests/modules/services/ssh-agent/linux/pkcs11-service-expected.service

@@ -3,6 +3,7 @@ WantedBy=default.target
 
 [Service]
 ExecStart=@openssh@/bin/ssh-agent -D -a %t/ssh-agent -P '/nix/store/*/lib,/usr/lib/libpkcs11.so,/usr/lib/other.so'
+SuccessExitStatus=2
 
 [Unit]
 Description=SSH authentication agent

tests/modules/services/ssh-agent/linux/timeout-service-expected.service

@@ -3,6 +3,7 @@ WantedBy=default.target
 
 [Service]
 ExecStart=@openssh@/bin/ssh-agent -D -a %t/ssh-agent -t 1337
+SuccessExitStatus=2
 
 [Unit]
 Description=SSH authentication agent