crypto: separate altname extensions with ", "

[indutny]

In newly introduced SafeX509ExtPrint I forgot to insert separators
between extensions, which lead to the "DNS:...DNS:..." thing for npm.

Fix #105

For @caineio:

1. yes
2. crypto
3. v0.12

[caineio]

Hello!

I am pleased to see your valuable contribution to this project. Would you
please mind answering a couple of questions to help me classify this submission
and/or gather required information for the core team members?

Questions:

1. Which part of core do you think it might be related to?
   One of: tls, crypto, buffer, http, https, assert, util, streams, smalloc, cluster, child_process, dgram, c++, docs, other (label)
2. Which versions of io.js do you think are affected by this?
   One of: v0.10, v0.12, v1.0.0 (label)
3. PR-only Does make test pass after applying this Pull Request.
   Expected: yes
4. PR-only Is the commit message properly formatted? (See
   CONTRIBUTING.md for more information)
   Expected: yes

Please provide the answers in an ordered list like this:

1. Answer for the first question
2. Answer for the second question
3. ...

Note that I am just a bot with a limited human-reply parsing abilities,
so please be very careful with numbers and don't skip the questions!

In case of success I will say: ...summoning the core team devs!.

In case of validation problem I will say: Sorry, but something is not right here:.

Truly yours,
Caine.

Responsibilities

1. indutny: crypto, tls, https, child_process, c++
2. trevnorris: buffer, http, https, smalloc
3. bnoordhuis: http, cluster, child_process, dgram

[indutny]

1. tls
2. v0.12
3. yes
4. yes

[caineio]

...summoning the core team devs!

[bnoordhuis]

LGTM

[indutny]

Landed in 687dfc9, thank you!