These syscalls are not necessary on their own, and they give the false
impression that a caller could set or get the thread name of any process
in the system, which is not true.
Therefore, move the functionality of these syscalls to be options in the
prctl syscall, which makes it abundantly clear that these operations
could only occur from a running thread in a process that sees other
threads in that process only.
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 :^)
Because these processes are expected to crash, generating a core dump
from them and throwing up a CrashReporter window is less helpful than
it is distracting while running many tests at a time.
Use the prctl for controlling the core dump-able flag to tell the kernel
we don't want core dumps from these processes. Note that because we also
build LibTest for Lagom, we have to check for MacOS which doesn't
support prctl(PR_SET_DUMPABLE).
