bladeRF-cli: Added --help-interactive command line argument

Added a new command line argument that prints out the help text for each
of the supported interactive commands. The purpose of this option is to
make it easier to generate man page documentation that incorporates the
interactive command help.
bladeRF-cli, doc: Use --help-interactive to get cmds from bladeRF-cli

When building the man page, use the --help-interactive option to
bladeRF-cli to get the list of interactive commands.
bladeRF-cli: Reformatted help text

Because of how man pages format things, it's easiest (maybe) to
use tabs and whatnot to designate indentation, etc, in the help
output.

This unfortunately makes some of the help output look relatively
crappy, but at least it is more consistent!  :-)

Additional fixes/tweaks will ensue.
bladeRF-cli: Initialized value for help-interactive option flag

Previously, I hadn't explicitly initialized the flag used to indicate
whether the --help-interactive command line option had been provided,
which would sometimes cause the --help-interactive output to be
displayed even when the option was not specified. In particular, when
help2man would execute the --version option, it would sometimes instead
get the --help-interactive output, which was obviously no good.
bladeRF-cli: Removed python references and build dependencies

Since the man page generation has been changed from a python script to a
bash script, there is no longer a dependency on python.
libbladeRF: Moved internal/external sampling logic to lms.c

The "top level" API function implementations in bladerf.c are intended
to be fairly thin to allow for some future efforts:
    - Pulling files such as lms.c and si5338.c into NIOS code
    - Wrapping per-device locks around calls in bladeRF.c in some
      upcoming thread safety changes.

As such, the logic involved with switching between internal & external
sampling has been relocated to lms.c

This addresses item #234.
host: Added additional configuration-time libusb function checks

Added checks for the following functions so that builds on older setups
will error out with error messages at configuration time, rather than
compilation errors at build time.
    - libusb_error_name()
    - libusb_get_device_speed()
    - libusb_handle_events_timeout()
    - libusb_handle_events_timeout_completed()
WIP: Modify the RXVGA2 calibration routines.
Calibration update.
libbladeRF: Moved LPF DC tuning value write into address loop

This is just for readability. There's no functional change, since
there's only 1 address to loop over anyway.
bladeRF-cli: Workaround - Added retry loop to TXLPF calibration

The TXLPF calibration appears to fail on the first two attempts to
run it. As a temporary workaround, a retry loop has been added
around attempts to run this calibration.
libbladeRF: Stubbed out autocal test program
Use linear interp to get first guess at DC offset values
Integer interpolation and apply rx auto cal parameters
Touched up output