Target modifiers fix for bool flags without value
Fixed support of boolean flags without values: `-Zbool-flag` is now consistent with `-Zbool-flag=true` in another crate.
When flag is explicitly set to default value, target modifier will not be set in crate metainfo (`-Zflag=false` when `false` is a default value for the flag).
Improved error notification when target modifier flag is absent in a crate ("-Zflag unset").
Example:
```
note: `-Zreg-struct-return=true` in this crate is incompatible with unset `-Zreg-struct-return` in dependency `default_reg_struct_return`
```
