MFC r200943: sh: Remove setting variables from dotcmd/exportcmd.

MFC r200943: sh: Remove setting variables from dotcmd/exportcmd.

It is already done by evalcommand(), unless special-ness has been removed,
in which case variable assignments should not persist. (These are currently
always special builtins, but this may change later: command builtin,
command substitution.)

This also fixes a memory leak when calling . with variable assignments.

Example:
  valgrind --leak-check=full sh -c 'x=1 . /dev/null; x=2'

git-svn-id: svn://svn.freebsd.org/base/stable/8@206944 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f