Add regen/uconfig_h.pl to regenerate uconfig.h

Add regen/uconfig_h.pl to regenerate uconfig.h

Most of the work is done by config_h.SH, but the wrapper is needed to add the
digest lines, so that t/porting/regen.t can verify that it's up to date.
I think that we need to take the "digest" approach, rather than "build to a
temporary file during testing", as we can't rely on a working Unix shell on
all platforms, and we couldn't even be sure that we get the skip list correct.