Moved global constants to class constants
Missed a couple SYSTEM_ prefixes
Quote shell argument with escapeshellarg()

Currently, the call to "scp -t" or "scp -f" just uses naive quoting - i.e. a couple of quote marks are thrown in.

But, this can easily be escaped from - if the filename has a quote mark of its own in it, for example.

e.g. if the filename is as follows, then bad things will happen:

 ";rm -rf /

Instead, escapeshellarg should be used, to make sure it gets escaped properly.
SCP: Add missing space after case statement.
Added SSH_AGENT_FAILURE constant back in
Merge pull request #528 from bantu/scp-case

SCP: Add missing space after case statement.

* bantu/scp-case:
  SCP: Add missing space after case statement.
Merge branch 'master' into php5

* master:
  SCP: Add missing space after case statement.
Merge pull request #527 from DavidAnderson684/patch-2

Quote shell argument with escapeshellarg()

* DavidAnderson684/patch-2:
  Quote shell argument with escapeshellarg()
Merge branch 'master' into php5

* master:
  Quote shell argument with escapeshellarg()
Merge pull request #524 from cnelissen/SystemPackageConstantsFix

System package constants fix

* cnelissen/SystemPackageConstantsFix:
  Added SSH_AGENT_FAILURE constant back in
  Missed a couple SYSTEM_ prefixes
  Moved global constants to class constants
Moved global constants to class constants and updated references where appropriate
Rebase
Merge branch 'NetPackageConstantsFix' of github.com:cnelissen/phpseclib into NetPackageConstantsFix