services: lsh: Add graceful handling of daemonic option.

* doc/guix.texi: Mention use case.
* gnu/services/ssh.scm (lsh-service): New #:keys (daemonic?, pid-file?,
  pid-file).  Build new lshd-command and expand service-requirement
  field.
Merge branch 'lsh' of github.com:Nebulieu/guix into lsh

Conflicts:
	doc/guix.texi
	gnu/services/ssh.scm