iio_attr: accept negative numbers as data to write to

[rgetz]

getopt doesn't understand the difference between the '-' before an
option and the '-' before a negative number. It can not - they are both
the same, and in some applications option zero (-0) might be valid.

in iio_attr this causes problems when we try to write negative numbers

root@analog:~# iio_attr -o -c adrv9002-phy voltage0 hardwaregain -10
iio_attr: invalid option -- '1'

Since the only time we do this in in iio_attr, handle the last option
differently (if the last option is a negative number).

This does mean we still don't pass -foo to an attribute to be written,
since iio_attr thinks that is an unknown "-f" option.

./tests/iio_attr -v -a usb -o -c ad9361-phy voltage0 hardwaregain -foo
Using auto-detected IIO context at URI "usb:3.39.5"
./tests/iio_attr: invalid option -- 'f'

but this does fix the reported bug #573

./tests/iio_attr -v -a usb -o -c ad9361-phy voltage0 hardwaregain -1
Using auto-detected IIO context at URI "usb:3.39.5"
dev 'ad9361-phy', channel 'voltage0' (output), attr 'hardwaregain', value '-10.000000 dB'
wrote 3 bytes to hardwaregain
dev 'ad9361-phy', channel 'voltage0' (output), attr 'hardwaregain', value '-1.000000 dB'

Signed-off-by: Robin Getz [email protected]

[rgetz]

this was a simple rebase on master, to make sure that the windows version works.