Tweak and regress-test 'command -x' (re: 66e1d446)

Turns out the assumption I was operating on, that Linux and macOS
align arguments on 32 or 64 bit boundaries, is incorrect -- they
just need some extra bytes per argument. So we can use a bit more
of the arguments buffer on these systems than I thought.

src/cmd/ksh93/features/externs:
- Change the feature test to simply detect the # of extra bytes per
  argument needed. On *BSD and commercial Unices, ARG_EXTRA_BYTES
  shows as zero; on Linux and macOS (64-bit), this yields 8. On
  Linux (32-bit), this yields 4.

src/cmd/ksh93/sh/path.c: path_xargs():
- Do not try to calculate alignment, just add ARG_EXTRA_BYTES to
  each argument.
- Also add this when substracting the length of environment
  variables and leading and trailing static command arguments.

src/cmd/ksh93/tests/path.sh:
- Test command -v/-V with -x.
- Add a robust regression test for command -x.

src/cmd/ksh93/data/builtins.c, src/cmd/ksh93/sh.1:
- Tweak docs. Glob patterns also expand to multiple words.

src/cmd/ksh93/data/builtins.c

@@ -487,13 +487,14 @@ const char sh_optcommand[] =
 "[V?Equivalent to \bwhence \b-v\b \acmd\a [\aarg\a ...]].]"
 "[x?Search \acmd\a as an external command, bypassing built-ins. "
 	"If the \aarg\as include a word "
-	"such as \b\"$@\"\b or \b\"${array[@]]}\"\b "
+	"such as \b\"$@\"\b or \b\"*.txt\"\b "
 	"that expands to multiple arguments, "
 	"and the size of the expanded \aarg\a list "
 	"exceeds \bgetconf ARG_MAX\b bytes, "
 	"then \acmd\a will be run multiple times, "
 	"dividing the \aarg\as over the invocations. "
-	"Any \aarg\as that come before the first \b\"$@\"\b or similar, "
+	"Any \aarg\as that come before the first "
+	"word that expands to multiple arguments, "
 	"as well as any that follow the last such word, "
 	"are considered static and will be repeated for each invocation "
 	"so as to allow all invocations to use the same command options. "