Rename settings from "unit" to "linux".

db/install.php

doc/en/CAS/Optimising_Maxima.md

doc/en/Developer/Development_history.md

doc/en/Developer/Releasing.md

doc/en/Developer/Unit_tests.md

equivdemo.php

healthcheck.php

lang/en/qtype_stack.php

settings.php

stack/cas/connector.class.php

stack/cas/connector.linux.class.php

stack/cas/connectorhelper.class.php

stack/cas/installhelper.class.php

tests/fixtures/test_maxima_configuration.php