Fix several alias hashtable issues.

Fix several alias hashtable issues.

Convert to use the strdup keys hash.  Addresses can be converted back
and forth from intl to local forms.  This frees and recreates a new
addr->mailbox string, resulting in the hash table key being a dangling
pointer.

Change alias hash table insert/remove to ensure the address is in intl
form.  The alias menu (previously) converted address entries to local
form when performing a completion.  Even with the pointer issue fixed,
the entries may not be removed from the hash if the intl and local
forms are different.

Lastly, there is no reason for the alias menu to manually convert to
local form before writing the address to the output buffer.
rfc822_write_address() has a display parameter that will call
mutt_addr_for_display() instead when set.  Change to set the display
parameter and remove the conversion calls.

This last change obviates the first two changes, but they are a good idea
in any case.