Commit Briefs
Show only whitelisted commit headers in 'got log'
Generic way to deal with gpgsig header or extra headers added by some tools. All headers still available via 'got cat' ok stsp@
got_error_from_errno -> got_error_prefix_errno
also add got_error_prefix_errno2 and got_error_prefix_errno3 which should hopefully all be merged into a single function with variadic args (but can't alloc mem)