proc: use human-readable values for hidepid

The hidepid parameter values are becoming more and more and it becomes
difficult to remember what each new magic number means.

Backward compatibility is preserved since it is possible to specify
numerical value for the hidepid parameter. This does not break the
fsconfig since it is not possible to specify a numerical value through
it. All numeric values are converted to a string. The type
FSCONFIG_SET_BINARY cannot be used to indicate a numerical value.

Selftest has been added to verify this behavior.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>