[patch] refine make regen to be more selective

make regen will currently cause recompilation of everything.

this patch makes it more selective

1 - copies do_cksum from regen.pl to regen_libs.pl, and simplifies it.
    This includes a change to unpack arg, to use U rather than C.
    It has no apparent effect, but seems worth noting here.

2 - uses it in safer_rename() to be more selective

3 - various fixups in *.pl-s called by regen.pl
    (remove unlink()s so comparison between old and new works)


This may expose some missing dependencies, which were covered by 
brute-force regen

It should allow 'regen' target to be rolled into 'all'

• [patch] refine make regen to be more selective by Jim Cromie
• Re: [patch] refine make regen to be more selective by Nicholas Clark
• Re: [patch] refine make regen to be more selective by Jim Cromie
• Re: [patch] refine make regen to be more selective by Nicholas Clark
• Re: [patch] refine make regen to be more selective by Jim Cromie
• Re: [patch] refine make regen to be more selective by Ben Morrow
• Re: [patch] refine make regen to be more selective by Nicholas Clark
• Re: [patch] refine make regen to be more selective by Nicholas Clark
• RE: [patch] refine make regen to be more selective by Robin Barker
• Re: [patch] refine make regen to be more selective by Jim Cromie
• Re: [patch] refine make regen to be more selective by Nicholas Clark