Remove sha1.el and hex-util.el; fix hexrgb compilation

sha1.el was included in Emacs 22.1 (released in 2007) and has since
been replaced by native functions; there's no point in us including it
anymore.  hex-util.el was only used by sha1.el.

Newer versions of Automake (since 1.13) treat Emacs Lisp files in
subdirectories differently, which causes problems with hexrgb.el.  I
made it copy hexrgb.el into the top directory, and it seems to work
for me.

Update README.  New minimum version is 23.1, which is when dns.el
started supporting SRV queries.  (Still need to update TLS section.)