sha1sum verification of source files

[Auke Kok]

Hi all,

I just added sha1sum verification code of source files to theedge, and 
hope to merge it within lunar soonish. Please note that:

- you can use sha1: and md5: together, apart, only one, none or whatever 
you like
- the old core code gracefully ignores sha1: syntax if it doesn't have 
the code to handle it.
- proof-of-concept/test case is ccache/DETAILS, please take a look at it 
if you wonder how to use the sha1: syntax

happy verifying!

PS most likely devs will start using the sha1: code more often and 
obsolete md5sum verification in the future, perhaps even better hashes 
will arise. It is unknown how fast we will fully adopt sha1 in favour of 
md5 at the moment. (I bet noprox can write a damn nice script that 
replaces md5: with proper sha1: lines... in about 15 minutes)

sofar