Improve `--summary` (especially for EOS)

[axxel]

Details, see commit messages. Overall effect is this new output of gphoto2 --set-config capture=on --summary on an EOS camera:

[...]
Device Property Summary:
Model ID                  (d049 ro u32): 2147484546
Friendly Device Name      (d402 ro str): 'Canon EOS 5DS'
Perceived Device Type     (d407 ro u32): 1
Session Initiator Info    (d406 rw str): 'Unknown Initiator'
[Unknown Property]        (d303 ro u8 ): 1
Battery Level             (5001 ro u8 ): Enumeration [100,0,75,0,50] value: 99% (99)
AutoExposureMode          (d105 rw u16): Enumeration [] value: 3
FocusMode                 (d108 ro u32): Enumeration [0,2,1] value: 3
DriveMode                 (d106 rw u16): Enumeration [0,4,5,19,20,16,17] value: 0
MeteringMode              (d107 rw u8 ): Enumeration [3,4,1,5] value: 3
WhiteBalance              (d109 rw u8 ): Enumeration [0,23,1,8,2,3,4,5,6,9] value: 1
ColorTemperature          (d10a rw u32): Enumeration [2500,2600,2700,2800,2900,3000,3100,3200,3300,3400,3500,3600,3700,3800,3900,4000,4100,4200,4300,4400,4500,4600,4700,4800,4900,5000,5100,5200,5300,5400,5500,5600,5700,5800,5900,6000,6100,6200,6300,6400,6500,6600,6700,6800,6900,7000,7100,7200,7300,7400,7500,7600,7700,7800,7900,8000,8100,8200,8300,8400,8500,8600,8700,8800,8900,9000,9100,9200,9300,9400,9500,9600,9700,9800,9900,10000] value: 5000
WhiteBalanceAdjustA       (d10b rw i32): Enumeration [-9,-8,-7,-6,-5,-4,-3,-2,-1,0,1,2,3,4,5,6,7,8,9] value: 0
WhiteBalanceAdjustB       (d10c rw i32): Enumeration [-9,-8,-7,-6,-5,-4,-3,-2,-1,0,1,2,3,4,5,6,7,8,9] value: 0
WhiteBalanceXA            (d10d rw u32): Enumeration [0,1,2,3] value: 0
WhiteBalanceXB            (d10e rw u32): Enumeration [0,1,2,3] value: 0
ColorSpace                (d10f rw u16): Enumeration [1,2] value: 1
[...]