split: allow a different size limit for each part

[chokore]

wimlib-imagex split currently applies a single size limit to every part, which makes it impossible to span a WIM across media with unequal capacities.

Concrete case: a Windows 11 install.wim (~7400 MiB) that has to fit on two 4.7 GB DVDs, where the first disc also carries the boot files. The first part must stay under ~3467 MiB while the second one takes whatever is left. With wimlib-imagex split install.wim install.swm 3467, three parts come out, because every part is capped at 3467 MiB: ceil(7409 / 3467) = 3.

Changes:

• lib: new wimlib_split_ex(), which takes a per-part list of size limits plus a default limit for the parts beyond the list. wimlib_split() is now a thin wrapper over it and keeps its exact signature and behavior, so there is no breakage for existing callers.
• wimlib-imagex split: PART_SIZE now also accepts a comma-separated list, e.g.
  3467,rest,3467,* (first part <= 3467 MiB, second part takes the rest) ,3467,1350,2700 or 3467,1350 (the last value is repeated for any further parts). A single number behaves exactly as before, and rest / * means "unlimited".