libio: Implement internal function __libc_readline_unlocked

This is a variant of fgets which fails with ERANGE if the
buffer is too small, and the buffer length is given as an
argument of type size_t.

This function will be useful for implementing NSS file reading
operations.  Compared to a direct implementation using the public API,
it avoids an lseek system call in case the line terminator can be
found in the internal read buffer.

libio/ftello.c

@@ -24,12 +24,15 @@
    This exception applies to code released by its copyright holders
    in files containing the exception.  */
 
+/* We need to disable the redirect for __ftello64 for the alias
+   definitions below to work.  */
+#define __ftello64 __ftello64_disable
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <libioP.h>
 #include <errno.h>
 
-
 off_t
 __ftello (FILE *fp)
 {
@@ -61,4 +64,7 @@ weak_alias (__ftello, ftello)
 
 #ifdef __OFF_T_MATCHES_OFF64_T
 weak_alias (__ftello, ftello64)
+# undef __ftello64
+strong_alias (__ftello, __ftello64)
+libc_hidden_ver (__ftello, __ftello64)
 #endif