(docolon): Set re_syntax_options to a value that is compatible with

(docolon): Set re_syntax_options to a value that is compatible with
what POSIX requires.  Also, don't let anchors match
newline; this fixes an incompatibility with tradition and with POSIX.
Don't warn about leading ^.  POSIX says it is unspecified whether
^ is a special character, which means that implementations can
either treat it as special or not, but either way a warning is not
allowed (unless the regexp is otherwise invalid).  Instead, anchor
the expression but treat ^ as an anchor; this is the traditional
behavior (e.g., Solaris 10).
(eval4, eval3, eval2): Treat non-numeric args, division by zero,
and the like as invalid expressions (exit status 2), not as
failure of 'expr' (exit status 3).  This is more consistent with
how Solaris behaves.