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().

Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

MFC r302511, r302771, r302845:
mail(1): check for out of memory conditions when calling calloc(3).

Approved by: re (gjb)