- envFromEnv panicked on an inherited row without "=", which execve allows.
- Such rows now reach the command instead of being dropped.
- Repeated names keep the first, as getenv does.
- Values holding NUL are refused by line number; they already failed with 125.
- README: how the file differs from a sourced shell script, both ways.

Upgrading from before 18e98dc, shipped in v0.1.0: malformed lines are rejected
rather than skipped, so a file that used to work may now fail the whole run
with 125. An "export" prefix is the likely trigger.