libbladeRF: Dynamically load SHGetKnownFolderPath in file_ops.c

This addresses issue #305, where mingw builds were failing because
SHGetKnownFolderPath appears to be missing from libshell32.a.
bladeRF-cli: Centralized checking of device presence and FPGA loaded state

These checks now occur once before a command is executed.

A subsequent commit will remove checking from the implementation of
each command.
bladeRF-cli: Removed per-command device presence and FPGA state checks

These checks are now performed in a single location prior to command
execution (in cmd.c), as of 2cf4111aed816748b91c06518a46d6d89ab16fe4.
bladeRF-cli: Fixed copyright date

This adjustment reverts a mistake I introduced while staging the
patch for 2cf4111aed816748b91c06518a46d6d89ab16fe4.