fix: drop capability aliases

fix: drop capability aliases

`CapabilityAliases` was a list of alternative names (just one) for IMAP
capability strings.  This isn't necessary.  We can put the strings in
`Capabilities` and fix the enumeration.

```c
enum ImapCaps
{
  ...
  X_GM_EXT1,
  X_GM_ALT1 = X_GM_EXT1,
  ...
};
```