nodejs6: use bundled openssl

[yan12125]

Description

This is a preparation step for OpenSSL 1.1 migration. Node.js 6 does not
build with OpenSSL 1.1:

:info:build ../src/node_crypto.h:91:40: error: invalid application of 'sizeof' to an incomplete type 'SSL_CTX' (aka 'ssl_ctx_st')

I switched to bundled openssl instead of using the openssl10 port to
lower the maintenance burden. nodejs6 is EOL'ed upstream and will be
considered to be dropped once nodejs8 works on OS X 10.6, anyway.

nodejs8 is not touched as it works with OpenSSL 1.1:

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.14.4 18E226
Xcode 10.2.1 10E1001

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
  The https.request example at https://nodejs.org/api/https.html#https_https_request_options_callback works fine.

[macportsbot]

Notifying maintainers:
@ci42 for port nodejs6.

[macportsbot]

Travis Build #6318 Passed.

Lint results

--->  Verifying Portfile for nodejs6
--->  0 errors and 0 warnings found.

Port nodejs6 success on xcode10.2. Log
Port nodejs6 success on xcode9.4. Log
Port nodejs6 success on xcode8.3. Log