Extract getopt and friends to <bits/getopt.h>

Bug: http://b/28067717
Change-Id: If19653d0da8b964c43768aa1051f9c926b3c6acb
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index b6046be..725cef5 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -36,6 +36,7 @@
 #include <sys/sysconf.h>
 
 #include <bits/fcntl.h>
+#include <bits/getopt.h>
 #include <bits/ioctl.h>
 #include <bits/lockf.h>
 #include <bits/posix_limits.h>
@@ -207,10 +208,6 @@
 extern int brk(void* __addr);
 extern void* sbrk(ptrdiff_t __increment);
 
-extern int getopt(int __argc, char* const* __argv, const char* __argstring);
-extern char* optarg;
-extern int optind, opterr, optopt;
-
 extern int isatty(int __fd);
 extern char* ttyname(int __fd);
 extern int ttyname_r(int __fd, char* __buf, size_t __buflen) __INTRODUCED_IN(8);