Support both " and ' as string delimiters when parsing the command line

Entity names are enclosed in quotes. Support both double-quoted and
single-quoted strings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Print more detailed v4l parse error messages

The following errors usually resulted in the same 'Unable to parse
format' message:

- syntax error in format description
- the requested pixel code isn't supported

Add more detailed error messages to give the user a clue what is going
wrong.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Print v4l parser position on error

Most parser functions take a **endp argument to indicate the caller
where parsing has stopped. This is currently only used after parsing
something successfully. This patch sets **endp to the erroneous
position in the error case and prints its position after an error
has occured.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>