consistently return number of bytes read from stdio read backend

the stdio FILE read backend's return type is size_t, not ssize_t, and
all of the special (non-fd-backed) FILE types already return the
number of bytes read (zero) on error or eof. only __stdio_read leaked
a syscall error return into its return value.

fread had a workaround for this behavior going all the way back to the
original check-in. remove the workaround since it's no longer needed.

src/stdio/__stdio_read.c

@@ -12,7 +12,7 @@ size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 	cnt = syscall(SYS_readv, f->fd, iov, 2);
 	if (cnt <= 0) {
 		f->flags |= cnt ? F_ERR : F_EOF;
-		return cnt;
+		return 0;
 	}
 	if (cnt <= iov[0].iov_len) return cnt;
 	cnt -= iov[0].iov_len;

src/stdio/fread.c

@@ -25,7 +25,7 @@ size_t fread(void *restrict destv, size_t size, size_t nmemb, FILE *restrict f)
 	/* Read the remainder directly */
 	for (; l; l-=k, dest+=k) {
 		k = __toread(f) ? 0 : f->read(f, dest, l);
-		if (k+1<=1) {
+		if (!k) {
 			FUNLOCK(f);
 			return (len-l)/size;
 		}