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

Remove the const qualifier from iconv(3) to comply with POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/iconv.html

Adjust all code that calls iconv.

PR: 199099
Exp-run by: antoine
MFC after: 2 weeks

Replace the #define for "iconv" so it is for the function name instead of
a macro with parameters. Remove a __DECONST hack and add consts instead
for gnu libiconv API compatability. This makes it work with things like
devel/boost-libs that expects to use "iconv" as though it were a pointer.

Add libiconv based versions of *c16*() and *c32*().

I initially thought wchar_t was locale independent, but this seems to be
only the case on Linux. This means that we cannot depend on the *wc*()
routines to implement *c16*() and *c32*(). Instead, use the Citrus
libiconv that is part of libc.

I'll see if there is anything I can do to make the existing functions
somewhat useful in case the system is built without libiconv in the
nearby future. If not, I'll simply remove the broken implementations.

Reviewed by: jilles, gabor