Handle SIGTERM and friends

Properly stop sampling when we get SIGTERM or similar.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
fmcomms1: Don't segfault if we cant read out_altvoltage_1A_scale_available

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Add an option to force load a plugin

By setting the OSC_FORCE_LOAD environment variable it is possible to load a
plugin even though it's identify function returns false. This is useful for
testing. OSC_FORCE_LOAD can either contain a space separated list of plugins to
load or the string "all" in which case all plugins will be loaded.

E.g.
	OSC_FORCE_PLUGIN=fmcomms1 ./osc

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Merge branch 'master' of github.com:analogdevicesinc/iio-oscilloscope