inpcom.c: exclude command 'shell' from lower case letter transformation

reported by Dominique Michel
[Ngspice-users] gs is confused with the shell command

shell gs -dBATCH -dNOPAUSE -sDEVICE=png256 -sOutputFile="plot.png"

src/frontend/inpcom.c

@@ -838,6 +838,7 @@ inp_read(FILE *fp, int call_depth, char *dir_name, bool comfile, bool intfile)
                  !ciprefix(".inc", buffer) &&
                  !ciprefix("codemodel", buffer) &&
                  !ciprefix("echo", buffer) &&
+                 !ciprefix("shell", buffer) &&
                  !ciprefix("source", buffer) &&
                  !ciprefix("load", buffer)
                 )