We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
These functions are now implemented in terms of getpwent_r() which
allows us to remove two FIXMEs about global variable shenanigans.
I'm also adding tests for both APIs. :^)