There was a support request hinting that other applications
concurrently accessed the SIM and were messing up the card state while
pySim-shell was running.
Let's avoid such situations by opening the card/reader in EXCLUSIVE mode
by default. If somebody really has a special use case, they can now add
the --pcsc-shared flag to restore the legacy behavior (SHARED mode).
Change-Id: I90d887714b559a4604708d3c6dd23b5e05f40576