[$] The "rare write" mechanism

One of the ways to harden the kernel is by tightening permissions on memoryto write-protect as much run-time data as possible. This means thekernel makes some data structures read-only to prevent malicious oraccidental corruption. However, inevitably, most data structures needread/write access at some point. Because of this, a blanket read-onlypolicy for these structures wouldn't work. Therefore, we need a mechanism that keepssensitive data structures read-only when "at rest", but allows writes whenthe need arises.