libbase: add ConsumePrefix/ConsumeSuffix.

adb was already using ConsumePrefix, and now we have another would-be
user in cutils. (There appears to be one place in adb that should use
ConsumeSuffix, so I'm assuming we'll want that sooner or later.)

I've kept these inline because adb and google3's versions both were, and
I'm easily led.

Test: treehugger
Change-Id: I29d99032f6f6ccbfaefece59725db8afb02a4c87