Print an error when the MHDIR/tmp doesn't exist

This is better than a SEGV... :).  I probably need a script that is
"mhng-install", or something along those lines (like exists for NMH).
Don't read(0) from GNUTLS

GNUTLS will bail out if you read(0) from it, which probably makes
sense -- it's better than the deadlock that would otherwise ensue.
This just prints out a slightly nicer error message, the GNUTLS one
makes it look like the connection may have dropped.
Use "[Gmail]/All Mail", not "[Gmail]/Trash"

Note that this has the odd side-effect that all messages (even those
in my inbox and in sent mail) will show up in the trash, but I guess
that's not the end of the world.  At least I won't start losing
messages all the time...
Fix GMail SELECT logic to actually work

I didn't have GMail SELECT-handling logic that did anything
reasonable.  This now handles SELECT on both MH names and GMail names
-- you've just got to be sure that those names are distinct (which
shouldn't be a problem).
Automatically grow line_buffer when needed

If a line_buffer ends up searching for a line that's longer than its
current internal state this will simply grow the buffer rather than
bailing out like before.  This changes the contract a bit: calling
recharge_size() can change the output of recharge_buffer(), but I
guess that's all I can do...

Maybe it'd be best to have the recharge stuff be two methods
(transaction and commit) instead of three.
Read the current sequence number from SQLite

This allows me to store the current message index and retrieve it
later, which means I can do things like "next" and "prev".  Note that
I don't actually have any command-line argument parsing, so I can't
really use this yet.