[debugserver] Initialize logging earlier in the startup sequence

Prior to setting up logging, we have uses of RNBLogSTDERR and
RNBLogSTDOUT. These macros will dump to STDERR and STDOUT respectively
if debugserver has a tty. Otherwise, it uses _DNBLog, which will do
nothing if a logging function hasn't been set up. For example, if you
specify a log file that cannot be opened for any reason and you don't
have a tty, you have 0 insight into what happened.

rdar://105473133

Differential Revision: https://reviews.llvm.org/D144142