sh: Remove the "exp" builtin.

The "exp" builtin is undocumented, non-standard and not very useful.

If exp's return value is not used, something like
VAR=$(exp EXPRESSION)
is equivalent to
VAR=$((EXPRESSION))
except that errors in the expression are fatal and quoting special
characters is not needed in the latter case.

If exp's return value is used, something like
if exp EXPRESSION >/dev/null
can be replaced by
if [ $((EXPRESSION)) -ne 0 ]
with similar differences.

The exp-run showed that "let" is close enough to bash's and ksh's builtin
that removing it would break a few ports. Therefore, "let" remains in 9.x.

PR:		bin/104432
Exp-run done by: pav (with some other sh(1) changes)

bin/sh/arith_yacc.c

@@ -344,7 +344,7 @@ arith_t arith(const char *s)
  *  The exp(1) builtin.
  */
 int
-expcmd(int argc, char **argv)
+letcmd(int argc, char **argv)
 {
 	const char *p;
 	char *concat;

bin/sh/builtins.def

@@ -60,7 +60,7 @@ echocmd		echo
 evalcmd		-s eval
 execcmd		-s exec
 exitcmd		-s exit
-expcmd		exp let
+letcmd		let
 exportcmd	-s export -s readonly
 #exprcmd		expr
 falsecmd	false