Merge 73913 - When performing SSL client authentication on Windows via NSS, c...

Merge 73913 - When performing SSL client authentication on Windows via NSS, change the returned key type to use a PCERT_KEY_CONTEXT allocated via NSS's PORT_Alloc(), rather than an HCRYPTPROV, for native client certificate authentication. 

There are two reasons for doing this; first, a PCERT_KEY_CONTEXT lets us transmit a dwKeySpec, indicating whether to use the AT_KEYEXCHANGE or AT_SIGNATURE key for CryptoAPI keys. Second, a small piece of syntactic fluff, a PCERT_KEY_CONTEXT easily supports CNG keys for Vista+, which though not presently supported, is a TODO. 

TBR=rsleevi 
BUG=37560, 71748
TEST=Perform SSL client auth on Windows.