Implement DAP "hover" context

Implement DAP "hover" context

A DAP client can request that an expression be evaluated in "hover"
context, meaning that it should not cause side effects.  In gdb, this
can be implemented by temporarily setting a few "may-" parameters to
"off".

In order to make this work, I had to also change "may-write-registers"
so that it can be changed while the program is running.  I don't think
there was any reason for this prohibition in the first place.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30476