file/unzip: support preserving timestamps
file/zip, file/tar: add #:get-timestamp argument

Allows the recorded timestamp to be adjusted in a general way.
raco pkg: preserve/adjust timestamps when extracting/creating packages

Perserve and adjust timestamps in a way that helps make re-packing an
installed-from-built package produce an archive with the same checksum
as the original.
Makefile: refine support for building an installer from a site
distro-build: refine client--server split
fix syntax checking for `#%declare`