The MacOS `find` command seems to be case insensitive. So the current
`mailsync` script results in counting each local inbox folder thrice,
resulting in the wrong # of new mails the user is notified about.
The commit implements another `test` command for `$(uname)` to decide
which arguments should be passed to `find`.

_In ref. to :_
https://github.com/LukeSmithxyz/mutt-wizard/issues/157#issuecomment-494132981

The current test  did not terminate  the script in cases  when the user
already has a private GPG key,  but they'd not initialized the password
store.
This leads to an infinite loop in the `getpass()` function.

The commit fixes https://github.com/LukeSmithxyz/mutt-wizard/issues/204

Resolves https://github.com/LukeSmithxyz/mutt-wizard/issues/197.

Checking for a secret key with format `<foo@bar.com>` may not work for
people that store key IDs in .gpg-id. Removing the `<` and `>` makes it
work for both beople that use emails and people that use IDs.

If -a is passed along with "$@", mailsync will sync all mailboxes even
if only a single channel is passed in with "$@". This breaks the
semantics of the `o` macro in mutt which should only sync the current
mailbox that is currently being viewed in mutt.