`xkbcomp -C` mangles the names of virtual-modifier masks

Do, for example:

setxkbmap us -print | xkbcomp -C - - | grep 'vmod_\w*Mask' | less

The #define lines are okay, with vmod_NumLockMask, vmod_AltMask, and so on. But all the lines after that have things like vmod_NumLocMask, vmod_AlMask, vmod_LevelThreMask... They are all missing the final letter of the virtual modifier's name.

The problem is in src/xkbtext.c, around line 130:

    len = strlen(tmp) + 1;
    if (format == XkbCFile)
        len += 4;
    ...
    if (format == XkbCFile) {
        snprintf(rtrn, len, "vmod_%s", tmp);
    }

When producing a C-header file, 4 is added to the printing length, to make room for the prefix, but... the prefix vmod_ has 5 bytes. Oops.

A merge request is coming up.