tree-wide: port everything over to fflush_and_check()

Some places invoked fflush() directly with their own manual error
checking, let's unify all that by using fflush_and_check().

This also unifies the general error paths of fflush()+rename() file
writers.

src/basic/fileio.c

@@ -28,21 +28,15 @@
 #include "fileio.h"
 
 int write_string_stream(FILE *f, const char *line, bool enforce_newline) {
+
         assert(f);
         assert(line);
 
-        errno = 0;
-
         fputs(line, f);
         if (enforce_newline && !endswith(line, "\n"))
                 fputc('\n', f);
 
-        fflush(f);
-
-        if (ferror(f))
-                return errno ? -errno : -EIO;
-
-        return 0;
+        return fflush_and_check(f);
 }
 
 static int write_string_file_atomic(const char *fn, const char *line, bool enforce_newline) {