add is_citrix() and get_system()

[jzadra]

is_citrix() returns a logical. get_systems() returns a character value, one of: "linux", "citrix", "local".

[jzadra]

@ebenp Added the export and documentation as we discussed.

[ebenp]

Hi Jon,

I get the following error when running is_citrix(). I googled a bit, but didn't find the function. What's current_r_location?

> is_citrix()
Error in current_r_location() : 
  could not find function "current_r_location"
Called from: .transformer(expr, env)`

[jzadra]

Oops. That was a reference to what became get_system(). I just removed that line as it adds confusion to the logical output.