Stop using K&R C features that have been removed

[tueda]

C23 (N3054 (PDF)) has removed K&R1-style function declarations from its syntax (N2432 (PDF)). Maybe it's time to assume C89-compatible compilers are always available.

For example, MesPrint is defined in message.c as follows:

int
#ifdef ANSI
MesPrint(const char *fmt, ... )
#else
MesPrint(va_alist)
va_dcl
#endif
{
...

As a result, VSCode can't jump to MesPrint using "Go to Definition".

We also assumes <stdarg.h> is always available.

We still use ANSI to select the date and timer implementations, though probably no one tries to compile FORM for freestanding environments where <time.h> and similar headers are not available (except for WebAssembly?). This code may need to be refactored in the future.

Note that there are many other places where K&R1 coding style is used (e.g., return(0);), but that's a separate issue (see also #712).

[coveralls]

coverage: 53.349% (+0.009%) from 53.34%
when pulling 2c3df78 on tueda:pr/refactor/remove-pre-c89-legacy-code
into 73a6a14 on form-dev:master.