posix: Revert the removal of the crypt prototype from <unistd.h>

Many applications still rely on this prototype.  Rebuilds without
this prototype result in an implicit function declaration, which can
introduce security vulnerabilities due to 32-bit pointer truncation.

NEWS

@@ -52,7 +52,10 @@ Deprecated and removed features, and other changes affecting compatibility:
 * libcrypt has been removed from the GNU C Library.  The configure
   options "--enable-crypt" and "--enable-nss-crypt" are no longer
   available.  <crypt.h>, libcrypt.a, and libcrypt.so.1 will not be
-  installed, and <unistd.h> will not declare the crypt function.
+  installed.  For now <unistd.h> continues to declare the crypt
+  function by default, to avoid introducing vulnerabilities into
+  existing applications due to a missing prototype.  This declaration
+  is deprecated and may be removed in a future glibc release.
 
   The replacement for libcrypt is libxcrypt, maintained separately from
   GNU libc, but available under compatible licensing terms, and providing

posix/unistd.h

@@ -1150,6 +1150,19 @@ ssize_t copy_file_range (int __infd, __off64_t *__pinoff,
 extern int fdatasync (int __fildes);
 #endif /* Use POSIX199309 */
 
+#ifdef __USE_MISC
+/* One-way hash PHRASE, returning a string suitable for storage in the
+   user database.  SALT selects the one-way function to use, and
+   ensures that no two users' hashes are the same, even if they use
+   the same passphrase.  The return value points to static storage
+   which will be overwritten by the next call to crypt.
+
+   This declaration is deprecated; applications should include
+   <crypt.h> instead.  */
+extern char *crypt (const char *__key, const char *__salt)
+     __THROW __nonnull ((1, 2));
+#endif
+
 #ifdef	__USE_XOPEN
 /* Swab pairs bytes in the first N bytes of the area pointed to by
    FROM and copy the result to TO.  The value of TO must not be in the