um: Refactor deprecated strncpy to memcpy

Use `memcpy` since `console_buf` is not expected to be NUL-terminated
and it more accurately describes what is happening with the buffers
`console_buf` and `string` as per Kees' analysis [1].

Also mark char buffer as `__nonstring` as per Kees' suggestion [2].

This change now makes it more clear what this code does and that
`console_buf` is not expected to be NUL-terminated.

Link: https://lore.kernel.org/all/202308081708.D5ADC80F@keescook/ [1]
Link: https://github.com/KSPP/linux/issues/90 [2]
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Cc: linux-hardening@vger.kernel.org
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>