ffprobe: force display of missing fields, depending on writer.flags

Add flags field to Writer, and define the
WRITER_FLAG_DISPLAY_OPTIONAL_FIELDS flag which forces the display of
optional fields, for example non-available or invalid fields.

Also set the flag in the default writer.

This change is required as for some writers it is preferable to show all
fields (for example for avoiding a variable number of fields in CSV
output), while for other formats (e.g. JSON, XML) it is better to leave
invalid/unavailable fields unspecified and thus simplify the parsing
stage on the deserializer side.
