Do not set LANG=C in every child environment.

LANG does not have to be set and setting it to default to the default
does not add any value.
test-tables: allow sparse tables and check mapping for -1

Jan: test-tables fails on my system. The one it's failing on is:
    syscall: 222 → (null) → -1
... and indeed, our own tables should not have holes, but syscall
tables certainly might.