Revert r335693, r335694, r335695 by eadler.

They cause mail(1) to crash in certain scenarios.

PR: 230196
Reported by: Pete French <petefrench@ingresso.co.uk>

MFC r302776, r302799:

mail(1): Bring some fixes from other BSDs.

- Use varargs properly
- Use pid_t
- Better handling of error conditions on forked jobs.
- Some prototype and warning cleanups.

Fix missing forked job changes from r302776 in wait_child().

MFC r270256:
Always check the limits of array index variables before using them.

Obtained from: DragonFlyBSD

MFC r270256:
Always check the limits of array index variables before using them.

Obtained from: DragonFlyBSD