Add $- ~= i check as well as --no-activate override
Add $- ~= i check as well as --no-activate override

While the $TERM checks might be antiquated, $TERM needs to be set for
our color formatting to work, so they are still safe. Add the $- check
here too, just in case. This should make loading Liquidprompt for tests
quite safe.

This whole line should be super POSIX safe. This is because we need to
quietly exit without error if Liquidprompt is loaded in a
non-interactive shell, even ones that aren't supported by the rest of
Liquidprompt.

The "x" prefixes on the vars are probably not needed, but some older
shells might interpret a "-f" or similar as a flag to the "test" or "["
command instead of a string to compare. This ugly fix prevents that from
happening.

Also add the "${TERM-}" format so that if $TERM is not set while
running under 'set -u', we can exit quietly instead of crash.

nojhan/liquidprompt,laurentbardin/liquidprompt,pkkolos/liquidprompt,milouse/liquidprompt,jaesivsm/liquidprompt