expose zfs_flags as debug.zfs_flags r/w tunable and sysctl

This knob is purposefully hidden under debug.

MFC after:	5 days
Sponsored by:	HybridCluster
Add missing static keyword.

MFC after:	2 weeks
fix debug.zfs_flags sysctl description in r258638

Pointyhat to:	avg
MFC after:	3 days