Optimize terminal detection

Optimize terminal detection

dlogutil needs to check if it's printing to the terminal or not in order
to see if it needs to apply colors or not in the automatic color mode.
Previously, we checked this repeatedly on every log we printed. However,
whether we are printing to the terminal or not shouldn't change at all.
Therefore, we should only check this once, during setup, and then never
call isatty() again. This commit achieves that.

Change-Id: I2084a4efbfeab057f33028a4b298a929aa3d5851