Use foo from path when running sage --foo
Trac #12627: The spkg/bin/sage script engraves paths to executables

There is no need to make full paths explicit - it is even an hindrance
when one moves things around.

Since sage-env already sets PATH correctly, let's just use that.

The use of 'command' to test for the presence of an executable in the
path is portable, as documented in this
[http://stackoverflow.com/questions/592620/check-if-a-program-exists-
from-a-bash-script stackoverflow question].

URL: http://trac.sagemath.org/12627
Reported by: Snark
Ticket author(s): Julien Puydt, Volker Braun
Reviewer(s): R. Andrew Ohana