Use safe_strerror instead of strerror where possible
This provides threadsafety. Unfortunately, since libinproctrace.so
does not link to gnulib, we can't use it there, especially since it
still includes the gnulib headers (so it is difficult to directly
call the system strerror_r).
gdb/ChangeLog:
2019-11-26 Christian Biesinger <cbiesinger@google.com>