util piconfig: fix reading

also start by reading the error number to recover from any potential error already present.
driver pigcs: fix turning off the encoder after reading position

now tried on a hardware (E-861 + a C-867 without encoder). It seems it could work.

At least it doesn't seem to prevent the E-861 from working. Need to check on a full C-867.
doc: update backend documentation for dynamic back-end changes

Several new VAs, and some roattributes became VAs.
Merge branch 'master' of vpn.delmic.com:odemis