Scrap readrune(), introducing fgetrune()

Interface as proposed by cls, but internally rewritten after a few
considerations.
The code is much shorter and to the point, aligning itself with other
standard functions. It should also be much faster, which is not bad.

utf.h

@@ -59,6 +59,7 @@ int isxdigitrune(Rune);
 Rune tolowerrune(Rune);
 Rune toupperrune(Rune);
 
-int readrune(const char *, FILE *, Rune *);
+int fgetrune(Rune *, FILE *);
+int efgetrune(Rune *, FILE *, const char *);
 void writerune(const char *, FILE *, Rune *);
 int chartorunearr(const char*, Rune **);