`uname -s` was already being executed every run for *cygwin* detection.
I've consolidated all of the OS detection into a single function. This
also fixed the problem of running `uname -s` twice for the `alt`
command.
With the new functionality, when the 'alt' command is called (or automatically
triggered), any file with a name ending in '##yadm_tmpl' is treated as a jinja
template. The template is processed by envtpl and the result is written to a
file without the '##yadm_tmpl' name. The variables passed into the template
processing are
YADM_CLASS
YADM_OS
YADM_HOSTNAME
YADM_USER
These variables are set according to the normal rules for
CLASS, OS, HOSTNAME, and USER during the alt processing.
