gpg: default to --no-auto-key-retrieve.

* g10/gpg.c (main): remove KEYSERVER_AUTO_KEY_RETRIEVE from the
default keyserver options.
* doc/gpg.texi: document this change.

This is a partial reversion of
7e1fe791d188b078398bf83c9af992cb1bd2a4b3. Werner and i discussed it
earlier today, and came to the conclusion that:

• the risk of metadata leakage represented by a default --auto-key-retrieve, both in e-mail (as a "web bug") and in other contexts where GnuPG is used to verified signatures, is quite high.
• the advantages of --auto-key-retrieve (in terms of signature verification) can sometimes be achieved in other ways, such as when a signed message includes a copy of its own key.
• when those other ways are not useful, a graphical, user-facing application can still offer the user the opportunity to choose to fetch the key; or it can apply its own policy about when to set --auto-key-retrieve, without needing to affect the defaults.

Note that --auto-key-retrieve is specifically about signature
verification. Decisions about how and whether to look up a key during
message encryption are governed by --auto-key-locate. This change
does not touch the --auto-key-locate default of "local,wkd". The user
deliberately asking gpg to encrypt to an e-mail address is a different
scenario than having an incoming e-mail trigger a potentially unique
network request.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>