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

Because we call __printf_out() with a on-stack buffer, also call
__printf_flush() so we are sure it won't be referenced after we return.

MFC after: 2 weeks

Fix various issues in how %#T is handled:
- If precision is 0, don't print period followed by no digits.
- If precision is 0 stop printing units as soon as possible
(eg. if we have three years and five days and precision is 0
print only 3y5d).
- If precision is not 0, print all units (eg. 3y0d0h0m0s.00).

MFC after: 2 weeks

Make all three if conditions look similar by always initializing nsec
and moving the default initialization of prec into the else clause.
The clang static analyzer erroneously thought that nsec can be used
uninitialized here; it was not actually possible, but better to make
the code clearer. (Clang can't know that sprintf() won't modify *pi
behind the scenes.)

Remove spurious "union arg" from printf.h

Make sure to always print something in the alternate time format.

Add missing 's' suffix on alternate rendition of time.

Add an extensible version of our *printf(3) implementation to libc
on probationary terms: it may go away again if it transpires it is
a bad idea.

This extensible printf version will only be used if either
environment variable USE_XPRINTF is defined
or
one of the extension functions are called.
or
the global variable __use_xprintf is set greater than zero.

In all other cases our traditional printf implementation will
be used.

The extensible version is slower than the default printf, mostly
because less opportunity for combining I/O operation exists when
faced with extensions. The default printf on the other hand
is a bad case of spaghetti code.

The extension API has a GLIBC compatible part and a FreeBSD version
of same. The FreeBSD version exists because the GLIBC version may
run afoul of our FILE * locking in multithreaded programs and it
even further eliminate the opportunities for combining I/O operations.

Include three demo extensions which can be enabled if desired: time
(%T), hexdump (%H) and strvis (%V).

%T can format time_t (%T), struct timeval (%lT) and struct timespec (%llT)
in one of two human readable duration formats:
"%.3llT" -> "20349.245"
"%#.3llT" -> "5h39m9.245"

%H will hexdump a sequence of bytes and takes a pointer and a length
argument. The width specifies number of bytes per line.
"%4H" -> "65 72 20 65"
"%+4H" -> "0000 65 72 20 65"
"%#4H" -> "65 72 20 65 |er e|"
"%+#4H" -> "0000 65 72 20 65 |er e|"

%V will dump a string in strvis format.
"%V" -> "Hello\tWor\377ld" (C-style)
"%0V" -> "Hello\011Wor\377ld" (octal)
"%+V" -> "Hello%09Wor%FFld" (http-style)

Tests, comments, bugreports etc are most welcome.