cmdutils: Rename read_file to cmdutils_read_file

This symbol name clashes with a symbol in gnutls, if linking
statically to that library.

Signed-off-by: Martin Storsjö <martin@martin.st>

cmdutils.c

@@ -785,7 +785,7 @@ int read_yesno(void)
     return yesno;
 }
 
-int read_file(const char *filename, char **bufptr, size_t *size)
+int cmdutils_read_file(const char *filename, char **bufptr, size_t *size)
 {
     int ret;
     FILE *f = fopen(filename, "rb");