Derive lens name from 0xA434 and 0xA433 if other means fail (#5157)

Implemented for
- Pentax
- Canon
- "Unknown"

Also constified `toString()` methods in `rtexif`.

rtexif/rtexif.cc

@@ -1767,7 +1767,7 @@ std::string Tag::nameToString (int i)
     return buffer;
 }
 
-std::string Tag::valueToString ()
+std::string Tag::valueToString () const
 {
 
     if (attrib && attrib->interpreter) {
@@ -3439,7 +3439,7 @@ short int int2_to_signed (short unsigned int i)
  * <focal>-<focal>mm f/<aperture>-<aperture>
  * NB: no space between separator '-'; no space between focal length and 'mm'
  */
-bool extractLensInfo (std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal)
+bool extractLensInfo (const std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal)
 {
     minFocal = 0.0;
     maxFocal = 0.0;