pcsclite: Introduce PCSCLITE_HP_DROPDIR env var

This allows to override the directory where PCSC-Lite searches for USB
drivers at runtime and should make our NixOS module much more clean so
that we don't need to imperatively stitch together plugin directories
anymore.

I'm using a GNU extension for the ternary operator to avoid computing
getenv("PCSCLITE_HP_DROPDIR") twice:

https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals



Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @viric, @wkennington