Port more shell lint improvements from illumos and ksh93v- (#374)

This commit adds onto <https://github.com/ksh93/ksh/pull/353> by porting
over two additional improvements to the shell linter:

1) The changes in the aforementioned pull request were merged into
   illumos-gate with an additional change.[*] The illumos revision of
   the patch improved the warning for (( $foo = $? )) to specify '$foo'
   causes the warning.[**] Example:
      $ ksh -n -c '(( $? != $bar ))'
      ksh: warning: line 1: in '(( $? != $bar ))', using '$' as in '$bar' is slower and can introduce rounding errors
   While I was porting the illumos patch I did notice one problem. The
   string it uses from paramsub() skips over the initial '{' in
   '${var}', resulting in the warning printing '$var}' instead:
      $ ksh -n -c '(( ${.sh.pid} != $$ ))'
      ...  in '(( ${.sh.pid} != $$ ))', using '$' as in '$.sh.pid}' is slower  ...
   This was fixed by including the missing '{' in the string returned by
   paramsub for ${var} variables.

2) In ksh93v-, parsing x=$((expr)) with the shell linter will cause ksh
   to warn the user x=$((expr)) is slower than ((x=expr)). This
   improvement has been backported with a modified warning:
      # Result from this commit
      $ ksh -n -c 'x=$((1 + 2))'
      ksh: warning: line 1: x=$((1 + 2)) is slower than ((x=1 + 2))
      # Result from ksh93v-
      $ ksh93v -n -c 'x=$((1 + 2))'
      ksh93v: warning: line 1: ((x=1 + 2)) is more efficient than x=$((1 + 2))
   Minor note: the ksh93v- patch had an invalid use of memcmp; this
   version of the patch uses strncmp instead.

References:
be548e87bc
https://code.illumos.org/c/illumos-gate/+/1834/comment/65722363_22fdf8e7/

src/cmd/ksh93/include/lexstates.h

@@ -138,6 +138,7 @@ extern const char e_lexsyntax3[];
 extern const char e_lexsyntax4[];
 extern const char e_lexsyntax5[];
 extern const char e_lexwarnvar[];
+extern const char e_lexarithwarn[];
 extern const char e_lexobsolete1[];
 extern const char e_lexobsolete2[];
 extern const char e_lexobsolete3[];