toGNUCommandLine: Unwrap value if set by mkForce/mkOverride

mkForce/mkOverride place the desired content in the "content"
attribute of an attrset, often changing the type of the option.

By default "toGNUCommandLine" will now unwrap such a wrapped value
instead of failing, though overrides to "mkOption" option will
change this behavior and may add other support for attrsets.

Author: j6carey