GNU/Hurd uses its own "typedef enum __error_t_codes error_t;"
([glibc]/sysdeps/mach/hurd/bits/errno.h), contrary to the default
"typedef int error_t;" ([glibc]/stdlib/errno.h).
The Mach/Hurd RPCs return kern_return_t values, for which, upon assigning them
to an error_t variable, GCC in C++ mode tells us "error: invalid conversion
from 'kern_return_t {aka int}' to 'error_t {aka __error_t_codes}'". Instead of
casting all these RPC return values to "error_t", just use "kern_return_t"
variables: