Function: externstr
Section: programming/specific
C-Name: externstr
Prototype: s
Help: externstr(str): execute shell command str, and returns the result as a
 vector of GP strings, one component per output line.
Doc: the string \var{str} is the name of an external command (i.e.~one you
 would type from your UNIX shell prompt). This command is immediately run and
 its output is returned as a vector of GP strings, one component per output
 line.
