Update libsrtp to version 2.0

srtp/TODO

-TODO List
-
-1.4.1
-
-  - document which fields are in NBO/HBO, and check for consistency.
-
-  - move HAVE_U_LONG_LONG inside of datatypes.c, or some other
-    separate file
-
-  - re-write configure.in to make cross-compilation easier
-
-  - eliminate GENERIC_AESICM by generalizing the code a bit
-
-Older comments
-
-  - add tests for key_limit_t datatype 
-
-  - move octet_get_weight() from datatypes.c to math.c (any other
-    funcs?)
-
-Changes and additions planned
-
-  Make cipher and auth dealloc() functions zeroize the key-storage
-  areas before calling free().
-
-  Eliminate key_len from auth_init()
-
-  Doucument internal APIs (cipher, auth, srtp_protect, ...)
-
-
-SRTP options not (yet) included in this libaray:
-
- - the aes-f8-mode cipher
- - the Master Key Index
- - re-keying using the key derivation function (only the initial
-   use of the PRF has been implemented, as it's sufficient
-   for most uses)
-
-
-(OLD) PLANNED CHANGES
-
-   strip out test/lfsr.c
-
-   Write new documentation!!!
-
-   Fix the x86 assembly code in aes.c.
-
-   Eliminate /* DAM */ - there's one in srtp.c
-
-   Change debugging so that it can print more than one line.  Or perhaps
-   just change it so that a single check of the debug-enabled flag is
-   needed.
-
-   Improve interface between cipher and rdbx - perhaps generalize rdbx
-   into 'nonce' datatype.
-
-   Make rijndael_icm accept variable sized keys.
-
-   Add rdbx functions that allow different-sized explicit sequence
-   numbers to be used.
-
-   Write uniform byte-buffering code for PRFs, preferably as macros.
-
-   Consider eliminating low-level alloc functions in favor of len()
-   functions, so that there need not be multiple allocations within a
-   particular alloc() function.