SSH2: send KEXINIT packet and identification string first or last (for 1.0 branch)

[terrafrost]

This branch adds four new methods:

• sendIdentificationStringFirst()
• sendIdentificationStringLast()
• sendKEXINITFirst()
• sendKEXINITLast()

RFC4253 § 4.2 says "when the connection has been established, both sides MUST send an identification string" but it does not say which side should send it first.

RFC4253 § 7.1 says "key exchange begins by each side sending the [SSH_MSG_KEXINIT] packet" but again it does not say which side should send it first.

In theory, it shouldn't matter which side sends it first but in practice it sometimes does.

#1046 demonstrates that phpseclib sending the SSH_MSG_KEXINIT packet last can sometimes result in errors so, in lieu of any evidence that sending it first can result in errors, I've changed it so that it sends it first by default now. BUT if that does cause errors then phpseclib can be easily made to send it last.

https://stackoverflow.com/q/40430024/569976 demonstrates that phpseclib sending the identification packet last can result in errors but 535104c#commitcomment-23493058 demonstrates that sending it first can result in errors as well. This commit preserves the current behavior but makes it so that the current behavior can be overwritten.

[dvaeversted]

This solved an issue with connection errors against Cisco devices that we started experiencing after upgrading from a 0.X release of phpseclib, to 2.0.6

[terrafrost]

This has been merged.

[terrafrost]

It's interesting SSH_MSG_NEWKEYS hasn't caused anyone an issue.

Quoting RFC4253 § 7.3, "Key exchange ends by each side sending an SSH_MSG_NEWKEYS message".

phpseclib sends SSH_MSG_NEWKEYS first.