luci-base: rename getLocaltime to getUnixtime

luci-base: rename getLocaltime to getUnixtime

There appear to be no consumers of getLocaltime. One instance of a
callGetLocaltime uses the system info call whose localtime property uses
tm->tm_gmtoff which is "Seconds East of UTC" i.e. a unixtime value
with the timezone baked in to the value.

Sometimes we actually want Unixtime. So rename to clarify what this
function actually returns.

Signed-off-by: Paul Donald <[email protected]>