How to change (sub)key usage of a PGP key?

Question

gpg2 generates keys with one or several of the (S)igning, (E)ncryption, (C)ertification usages set. However, e.g. Enigmail creates a primary key also set for (A)uthentication, which GnuPG then shows. How can this be set/modified using gpg2? I can't find any command in --edit-keys, and when I create a subkey the only options are

 Please select what kind of key you want:  
   (3) DSA (sign only)  
   (4) RSA (sign only)  
   (5) Elgamal (encrypt only)  
   (6) RSA (encrypt only)

Answer 1

In order to add a new subkey with specific usages, start gpg2 with the --expert switch. Then the options are

Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
Your selection?

Where the last two methods allow toggling each of S, E, A individually. C is only possible for a primary key, the usage of which you can also set with gpg2 --expert --gen-key. I don't know of any way to modify the usage field at a later point however.