Move base64_decode_atom into misc.c.

I'm about to need to refer to it from a source file that won't
necessarily always be linked against sshpubk.c, so it needs to live
somewhere less specialist. Now it sits alongside base64_encode_atom
(already in misc.c for another reason), which is neater anyway.

[originally from svn r10218]

misc.h

@@ -44,6 +44,7 @@ int toint(unsigned);
 char *fgetline(FILE *fp);
 
 void base64_encode_atom(unsigned char *data, int n, char *out);
+int base64_decode_atom(char *atom, unsigned char *out);
 
 struct bufchain_granule;
 typedef struct bufchain_tag {