Split the Advanced OpenSSH section.

This allows us to link to specific sub-sections for specific operating
systems.

Closes #44.
Merge branch 'master' into sean/split-advanced-openssh

Conflicts:
	manual/remote-access/gnu-linux/ssh.rst
Merge branch 'master' into sean/split-advanced-openssh
Merge pull request #49 from seanfisk/sean/split-advanced-openssh

Split the Advanced OpenSSH section.