Unexport internal functions from sysinfo.h

Keep them in sysinfo_priv.h, so they still can be unit-tested.
Unexport internal function
Move IsInterfaceAddress to proper place
Implement gateway DetectEnvironment() function to hide libenv internals
Split LoadSecretKeys in two more coherent functions

Updating keys in ppkeys/ has nothing in common with
loading secret keys for the agent.
Unscrew {SHORT,STR}_CFENGINEPORT

Instead of storing a string and a short in network byte order
just store a plain int and use it everywhere.
Export ParseHostname to use in enterprise
Checking /joyent.*/ against uname.version should not touch $(match.N) vars

So use StringMatchFull instead of FullTextMatch