Verify payload length early on

[cgwalters]

I was looking at something else here and happened to notce we didn't seem to be sanity checking the length of strings in some places.

Looking harder, first thing I checked is a really long filename in a dump file; that's fine, we do check that.

Next I tried symlinks, and that caused a an assertion trip process abort (thankfully at least) deep in the bowels of the EROFS generation.

Fix this by checking the payload length against PATH_MAX.

Also:

• Add the missing error checking in mkcomposefs
• Add test infrastructure for "should fail" dumpfiles