#140 postread functions flood the log

[Markus Armbruster]

When pln_postread() detects a plane with a bad id, it
logs a complaint and zaps the plane. This assigns zero
to the plane id, which will trigger the same complaint
next time around (unless it happens to be plane#0).
This fills the log quickly with messages like this:

pln_postread: Error - 0 != <number>, zeroing.\n

Other postread functions are believed to have the same bug.

[Markus Armbruster]

Logged In: YES
user_id=596270

Work-around: overwrite the offending plane slot by building
enough planes. They can safely be scrapped right afterwards.

[Markus Armbruster]

Logged In: YES
user_id=596270

pln_postread() is called on the *copy* made by ef_read().
Nothing it does can fix the original, which remains broken
until something overwrites it with ef_write(). This feels
wrong.