Function: readstr
Section: programming/specific
C-Name: readstr
Prototype: D"",s,
Help: readstr({filename}): returns the vector of GP strings containing
the lines in filename.
Doc: Reads in the file \var{filename} and return a vector of GP strings,
each component containing one line from the file. If \var{filename} is
omitted, re-reads the last file that was fed into \kbd{gp}.