Sync sh(1) in stable/10 to head.

This is a MFC of all the commits listed below.

My original goal of this change was to only merge the move of the tests
from tools/regression/bin/ into the new layout (which include tests for
sh(1) and other tools as well).  However, doing so is tricky due to the
ongoing work in sh(1) and, especially, the many changes to its tests
since stable/10 was first branched.

Merging everything is the simplest way to achieve this goal and, as a
bonus point, we get various fixes and miscellaneous improvements into
the branch.

Per jilles' suggestion, I'm avoiding the merge of a couple of changes
(r256850 and r257506) that required depending kernel changes.  I'm also
avoiding very recent changes that have not had a long enough time to be
validated in current.

This is "make tinderbox" clean.

r256735	sh: Remove one syscall when waiting for a foreground job.
r257399	sh: Allow trapping SIGINT/SIGQUIT after ignore because of '&'.
r257504	sh: Reorder union node to reduce its size on 64-bit platforms.
r257920	sh: Add a test case for would-be assignments that are not due to quoting.
r257929	sh: Properly quote alias output from command -v.
r258489	sh: Add tests for the </dev/null implicit in a background command.
r258533	sh: Add more tests for the </dev/null implicit in a background command.
r258535	sh: Make <&0 disable the </dev/null implicit in a background command.
r258776	sh: Prefer memcpy() to strcpy() in most cases. Remove the scopy macro.
r259047	sh: Split set -x output into a separate function.
r259210	Migrate tools/regression/bin/ tests to the new layout.
r259844	sh: Remove an unused variable.
r259846	sh: Initialize OPTIND=1 even if it came from the environment.
r259874	sh: Simplify code related to PPID variable.
r259946	sh: Don't check input for non-whitespace if history is disabled.
r260246	sh(1): Discourage use of -e.
r260506	Run the sh(1) and test(1) tests as unprivileged.
r260586	Mark the bin/pax tests as requiring perl.
r260634	Use TAP_TESTS_PERL to register the legacy_test in bin/pax.
r260635	Replace hand-crafted Kyuafiles with automatic generation.
r260654	sh: Remove SIGWINCH handler and just check for resize before every read.
r261121	sh: Add test for nested alias.
r261125	sh: Solve the alias recursion problem in a less hackish way.
r261141	sh: Do not depend on parse/execute split in new alias test.
r261160	sh: Add tests for alias names after another alias.
r261192	sh: Allow aliases to force alias substitution on the following word.
r262533	sh: Make expari() static.
r262565	sh: Do not corrupt internal representation if LINENO inner expansion fails.
r262697	sh: Simplify expari().

Reviewed by:	jilles