[v17] Read ~/.tsh/environment as the target user

[r0mant]

This PR adds a readfile sub-command that can be reexec'd in order to read a file as a target user. This is useful in cases where we need to do some processing on file contents as root, but we want to make sure the target user has access to the file at the time of reading it (e.g. to prevent arbitrary file reads as root on behalf of the user). It works by passing an os.Pipe to the subcommand in order to send the file contents back to the parent process. To support future use cases, ProcessFileAsUser is generic and accepts a process() callback responsible for reading file contents from an io.Reader and marshaling into some type T.

changelog: Fixed security issue with arbitrary file reads on SSH nodes.