PLUGINS: Add the ability to look up symbols

Signed-off-by: Robin Getz <robin.getz@analog.com>
OSC: print the env vars looked at during help, so I don't need
	to look in the source all the time

Signed-off-by: Robin Getz <robin.getz@analog.com>
Merge branch 'master' of https://github.com/analogdevicesinc/iio-oscilloscope

Fix osc.c
SCPI: make more things static so they don't bleed into other files
      allow rx (spectrum analyzer) functions to be called from other
      plugins
FMCOMMS1: Start looking at cal'ing Tx side
          Be a little smarter about Rx side handling