remove unused variable from ssl3_get_client_hello

ok tedu@ miod@
Fix a double free in a can't-fail error path in PKCS7_decrypt(), by removing
the error path altogether and simplifying the local variables as a result.
joint work with jsing@; ok jsing@ tedu@
KNF
Upon realloc() failure, free() the original pointer and remove the stupid
comments implying you don't have to.
ok tedu@
Simplify realloc() usage; ok tedu@
Use size_t as realloc() size argument whenever possible. ok tedu@
OPENSSL_stderr() is unused so nuke it. OPENSSL_showfatal() is only used by
OpenSSLDie(), which is in the same file, so just make it static.

ok miod@
Explicitly include <openssl/opensslconf.h> in every file that references
an OPENSSL_NO_* define. This avoids relying on something else pulling it
in for us, plus it fixes several cases where the #ifndef OPENSSL_NO_XYZ is
never going to do anything, since OPENSSL_NO_XYZ will never defined, due
to the fact that opensslconf.h has not been included.

This also includes some miscellaneous sorting/tidying of headers.