Principle of least surprise: make CMAC_CTX_free(), OCSP_REQ_CTX_free() and
X509_STORE_CTX_free() accept NULL pointers as input without dereferencing them,
like all the other well-behaved *_CTX_free() functions do.
guard inclusion of sys/sysctl.h so we can detect at compile time and
keep linux distros happy that don't have it.
ok bcook@