libkiconv: address memory leak in not-found cases

Found in "Understanding and Detecting Disordered Error Handling with
Precise Function Pairing" by Qiushi Wu et al.

Reviewed by: imp, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29896

(cherry picked from commit 3cfd08c1c74058451a02bac35307bf7fa509c617)

Git Hash: 8e64d97d94f7a1ad186c1168e905f18eb2fe5f8b
Git Author: emaste@FreeBSD.org

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

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

free(3) won't mess with errno so return it as-is.

Submitted by: Jaakko Heinonen <jh saunalahti fi>

Split tolower/toupper code from usual xlat16 kiconv table, and make it
possible to do tolower/toupper independently without code conversion.

Submitted by: imura (but bugs are mine)
Obtained from: http://people.freebsd.org/~imura/kiconv/
(1_kiconv_wctype_kern.diff, 1_kiconv_wctype_user.diff)