[Patch] Add OSX Keychain support

[Oliver R. <oli...@gm...>]

Howdy, isync folks.

I've attached a patch to support direct OSX Keychain access on OSX,
which adds two things:
- new config "KeychainName" and "KeychainAccount" to look up a
specific generic password in the Keychain
- an attempt to look up the Internet password for the account's host,
user, and protocol (SMTP), which doesn't need configuration

If neither finds a password, then the usual password retrieval path is taken.

Rationale:
I used PassCmd, but neither `gpg -d .authinfo.gpg` nor `security
find-generic-password [...]` are a good choice in my eyes, because I'd
have to type in my local password (GPG or Keychain access) each time
or add a rule to cache it in gpg-agent or always allow security access
to the Keychain. The latter options allow anyone who gains access to
my machine to retrieve my password on the command line.

With the Keychain access directly from mbsync, however, Keychain can
do the access control based on the process's binary, shutting down
this particular attack possibility.

In particular, the config for a generic password works very well if
there already is an entry for it in the Keychain, which gets updated
when the password changes, and other programs need to access the same
password, which they can access in the same way, in my case this is
"msmtp" to send email.

Let me know whether this you'd consider merging this or if it needs changes.

Toodle-pip
  Oliver

PS: this sort of thing could also be done with the GNOME Keyring or KDE Wallet