Fix conversion warnings in libdispatch

[ZedThree]

With this PR, there will be just under 400 warnings left, down from 2400 in November. Here's a summary of what's left:

Warnings	Directory
261	ncgen (see #2897)
119	libnczarr (see #2852)
75	libdap2
63	ncgen3 (see #2900)
31	libdap4 (see #2898)
27	include
23	libhdf5
13	libsrc
9	ncdump
6	build/libsrc
1	libdispatch (see below)
1	nc_test
1	oc2
386	Total

---

There's one remaining warning in libdispatch, and while it's an easy fix, it looks pretty suspicious:

netcdf-c/libdispatch/daux.c: In function ‘computefieldinfo’:
netcdf-c/libdispatch/daux.c:291:37: warning: conversion to ‘size_t’ {aka ‘long unsigned int’} from ‘int’ may change the sign of the result [-Wsign-conversion]
  291 |         offset += getpadding(offset,alignment);
      |                                     ^~~~~~~~~

and here's the context:

    for(offset=0,i=0;i<cmpd->nfields;i++) {
        struct NCAUX_FIELD* field = &cmpd->fields[i];
	int alignment = 0;
	nc_type firsttype = findfirstfield(cmpd->ncid,field->fieldtype);

        /* only support 'c' alignment for now*/
	switch (field->fieldtype) {
	case NC_OPAQUE:
	    field->alignment = 1;
	    break;
	case NC_ENUM:
	    status = ncaux_type_alignment(firsttype,cmpd->ncid,&field->alignment);
	    break;
	case NC_VLEN: /*fall thru*/
	case NC_COMPOUND:
            status = ncaux_type_alignment(firsttype,cmpd->ncid,&field->alignment);
	    break;
	default:
            status = ncaux_type_alignment(field->fieldtype,cmpd->ncid,&field->alignment);
	    break;

	}
        offset += getpadding(offset,alignment);
        field->offset = offset;
        offset += field->size;
    }

Notice how alignment is initialised to zero and then never changed? This means that getpadding(...) always returns zero. Should this actually be offset += getpadding(offset, field->alignment)?

[WardF]

Merged the change with the Github-hosted Windows Runner test; it should be a good way to test the waters, and will also lay the groundwork for getting this in so that we can get 4.7.3 out.