sh: Expand assignment-like words specially for export/readonly/local.

sh: Expand assignment-like words specially for export/readonly/local.

Examples:
  export x=~
now expands the tilde
  local y=$1
is now safe, even if $1 contains IFS characters or metacharacters.

For a word to "look like an assignment", it must start with a name followed
by an equals sign, none of which may be quoted.

The special treatment applies when the first word (potentially after
"command") is "export", "readonly" or "local". There may be quoting
characters but no expansions. If "local" is overridden with a function there
is no special treatment ("export" and "readonly" cannot be overridden with a
function).

If things like
  local arr=(1 2 3)
are ever allowed in the future, they cannot call a "local" function. This
would either be a run-time error or it would call the builtin.

This matches Austin Group bug #351, planned for the next issue of POSIX.1.

PR:		bin/166771