Fix inconsistent implementations of realpath in klee-uclibc

[lipeng28]

Hi There

I found there exist some incomplete or unsupported functions in klee-uclibc during my testing with KLEE, I created a simple patch based on current klee-uclibc repo, please check.

1. Inconsistent implementation with realpath of uclibc
   when I tested programs containing realpath functions, KLEE failed to interpret it correctly, then I found it is caused by obsolete realpath function, so I incorporated the current implementation of realpath in uclibc into klee-uclibc. It works well now.

2. libc/string/strdup.c does not include __strdup implementation
   When I tested binutils, I found that strdup call will be compiled to __strdup which is interpreted incorrectly by KLEE, so I added the implementation based on Wstrdup.

Thanks
Peng

[ccadar]

Hi @lipeng28, thanks again for your contribution. Can I ask you two things:
(1) Could you split this into two different PRs, since the two changes are unrelated.
(2) Can you add a test case in each case showing the problem that your change is fixing?