map: fail if key is currently executing

If during execution of a mapping, that same mapping is replaced,
there is undefined behavior because we destroy a mapping that we are
still iterating over.

I have been using this mapping inside my kakrc to re-source the kakrc.

	map global user s %{:source "%val{config}/kakrc"<ret>} -docstring 'source "%val{config}/kakrc"'

Now <space>s happens to not trigger undefined behavior because the
mapping stays the same.

However it triggers an assertion added by Commit e49c0fb04 (unmap:
fail if the mapping is currently executing, 2023-05-14), specifically
the destructor of ScopedSetBool that guards mapping execution.

Fix these by banning map of a key that is executing, just like we
did for unmap.

Alternative solution: we could allow mapping (and even unmapping)
keys at any time and keep them alive by moving them into a trash can,
like we do for clients and others.

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