env: considerably beef up environment cleaning logic

Now, actually check if the environment variable names and values used
are valid, before accepting them. With this in place are at some places
more rigid than POSIX, and less rigid at others. For example, this code
allows lower-case environment variables (which POSIX suggests not to
use), but it will not allow non-UTF8 variable values.

All in all this should be a good middle ground of what to allow and what
not to allow as environment variables.

(This also splits out all environment related calls into env-util.[ch])
test-env-replace: better user assert_se() everywhere

For test tools it's better to use assert_se() rather than assert(),
since the former is not optimized away, even if -DNDEBUG is used. That
means the test program now retains its usefulness even if -DNDEBUG is
used.

This also adds in some OOM checks, to be extra anal...
util: introduce FOREACH_LINE for iterating through files
path-util: fix memory leak
util: rework load_env_file()

Inner library calls should not invoke log_oom(), that's something for
main programs, not library calls.

Don't read through uninitialized memory if a file ends in a continuation
line.

Add comments for the non-obvious bits.

Don't choke on comment lines that are continuation lines.

Simplify some things.