#635 New syntax for "call" ARGC + ARG0 + ARG1...ARG9

[Ethan Merritt]

I've been struggling to update the "call" syntax to something more compatible with current gnuplot capabilities. The original syntax performs macro-like expansion triggered by the character sequences $0 ... $9, with the count of parameters substituted for $#.

Problems with the original syntax:
1) Consumes quote marks on input, ignores quotes during macro expansion
2) Because of (1) it handles string variables and string constants poorly
3) The $# expansion has been broken for years because it looks like a comment
4) This use of $N conflicts with the use of $N as shorthand for column(N)
5) The use of $ and the peculiar handling of quotes makes it very hard to generate or pass through lines from a unix-like shell script

This patch introduces an alternative syntax for the "call" arguments. They are loaded into ordinary gnuplot variables that are saved/restored across the call.
ARG0 = name of the script being executed
ARGC = count of parameters passed to the script
ARG1..ARG9 = the arguments stored as strings

Notes:
- Because the arguments are constructed and accessed as regular string variables, you have the choice of using them directly (e.g. set title ARG3) or expanding them as macros (e.g. plot @ARG3). The former is usually appropriate for strings, the latter is usually appropriate for numerical values or expressions.
- ARG0 holds the name of the script being executed during "load" also, as well as during execution of files named on the gnuplot command line.

The patch contains a simple demo "calltest.dem" that illustrates this.

[Ethan Merritt]

Here's the output of running the demo script:

stonelion [1188] ./gnuplot calltest.dem
Entering calltest.dem with 0 parameters
Now Exercise the call mechanism
Entering calltest.dem with 5 parameters

        Variables beginning with ARG:
        ARGC = 5
        ARG0 = "calltest.dem"
        ARG1 = "1.23e4"
        ARG2 = "string constant"
        ARG3 = "FOO"
        ARG4 = "BAZ"
        ARG5 = "3 + log(BAZ)"
        ARG6 = ""
        ARG7 = ""
        ARG8 = ""
        ARG9 = ""

ARG1 (numerical constant) came through as 1.23e4
     @ARG1 = 12300.0
     (ARG1 == @ARG1) is TRUE
ARG2 (string constant) came through as string constant
     words(ARG2) = 2
ARG3 (undefined variable FOO) came through as FOO
ARG4 (numerical variable BAZ=5.67) came through as BAZ
     @ARG4 = 5.67
ARG5 (quoted expression) came through as 3 + log(BAZ)
     @ARG5 = 4.73518911773966

[Ethan Merritt]

fix lf_pop of undefined variable