save-entropy(8), rc.d/random: Set nodump flag

Tag saved entropy files as "nodump," to signal that the files should not be
backed up by dump(8) or other automated backup software that honors the file
flag.

Do not produce an error if the target file resides on a filesystem that does
not support file flags (e.g., msdos /boot).

Reviewed by: delphij
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20358