crypto: createDiffieHellman throw for bad args

Previously crypto.createDiffieHellman() would fail silently when a bad
argument was passed for prime/prime_length. Now throws TypeError.

Fixes: https://github.com/joyent/node/issues/8480
Signed-off-by: Trevor Norris <trev.norris@gmail.com>
doc: fix wording and punctuation in modules

[trev.norris@gmail.com: break lines at 80 characters]
Signed-off-by: Trevor Norris <trev.norris@gmail.com>
test: fix test-child-process-spawn-typeerror

You cannot spawn 'dir' on Windows because it's not an executable.  Also,
some people might have 'ls' on their path on Windows, so I changed
invalidCmd to something that's highly unlikely to exist.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
doc: correct createSecureContext

Remove incorrect stablity indication of tls.createSecureContext, and
format stablity indication of crypto.createCredentials.

Fixes: e50749 "doc: document `tls.createSecureContext`"
Reviewed-by: Trevor Norris <trev.norris@gmail.com>