process: return ESRCH when a PID is not valid anymore

so far, when we read something from /proc/$PID we would pass on the
ENOENT from the kernel as error, if the process was missing. With this
change we systematically convert this to ESRCH, which is the more
appropriate error code, and what all the other glibc/syscalls like
kill() use.

All code that calls these functions should be fine with this change. In
fact, one invocation of get_process_exe() in bus-creds.c already assumed
ESRCH would be returned if a process is missing, and this assumption is
now validated after the change.