Improve support of name spaces in libm4. Put the m4 builtins

Improve support of name spaces in libm4.  Put the m4 builtins
in the `libm4' name space.

* libm4 (m4_changequote, m4_define, m4_defn, m4_dnl, m4_indir,
m4_undefine): Private copies in the global name space.  Used by
the name space mechanisms.
(m4_namespace_register): New macro.
(m4_namespace_define): Use it.
(builtin, changequote, defn, dnl, esyscmd, ifdef, ifelse, indir,
patsubst, popdef, pushdef, regexp, undefine, syscmd, sysval): Put
in the `libm4' name space.
(m4_disable, m4_enable): Use the private m4_indir, otherwise once
`libm4' closed, there is no `indir' available, and therefore no
means to reopen a name space.