MFC r351213: assert that td_lk_slocks is not leaked upon return from kernel

MFC r331490:
Account the size of the vslock-ed memory by the thread.

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 r314253:
Do not leak mount references for dying threads.

MFC r302614:
Revive the check, disabled in r197963.

MFC r302999:
On first exec after vfork(), call signotify() to handle pending
reenabled signals.

Approved by: re (delphij)