fileio: add new helper write_base64_file_at() which encodes a binary object into base64 and writes it to a file

src/basic/fileio.h

@@ -66,6 +66,8 @@ static inline int write_string_file(const char *fn, const char *line, WriteStrin
         return write_string_file_ts(fn, line, flags, NULL);
 }
 
+int write_base64_file_at(int dir_fd, const char *fn, const struct iovec *data, WriteStringFileFlags flags);
+
 int write_string_filef(const char *fn, WriteStringFileFlags flags, const char *format, ...) _printf_(3, 4);
 
 int read_one_line_file_at(int dir_fd, const char *filename, char **ret);