spec: clarify if a type with underlying unsafe.Pointer type can be used for unsafe conversions

[griesemer]

See also #6326 and #18130 (comment) .

In the same vein, decide if type P = unsafe.Pointer should be allowed.

[rsc]

If gc and gccgo allow it today, it should continue to be allowed. We've passed the point of no return on a breaking change like that. The "audit" argument is fairly weak: the audit will find the initial definitions and then the people auditing can decide easily enough whether to expand their search.

[gopherbot]

CL https://golang.org/cl/42132 mentions this issue.