Raise limit for passwd and shadow entry length

Moreover, include checks to prevent writing entries longer than the
length limit.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1422497

Signed-off-by: Tomáš Mráz <tm@t8m.info>
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

lib/defines.h

@@ -327,6 +327,9 @@ extern char *strerror ();
 # endif
 #endif
 
+/* Maximum length of passwd entry */
+#define PASSWD_ENTRY_MAX_LENGTH 32768
+
 #ifdef HAVE_SECURE_GETENV
 #  define shadow_getenv(name) secure_getenv(name)
 # else