- add OS X privsep user creation instructions from jasper@
- update install instructions to be the same everywhere
- remove outdated compiler flags

Allow overriding the user and the config file paths for ntpd.
This also adds back --privsep-path from OpenNTPD, which really only
changes the installation instructions from 'make install', since ntpd
always uses the home directory of the ntpd user.
This borrows from the Darren's portable OpenNTPD and from OpenSSL, add
credits and a license.

This makes it easier to see what is supported by each OS, and to improve
them individually.

Well, support may be too concrete of a term. There is a lot that is
missing in OS X, and not everything can be implemented with portability
shims. The time APIs in OS X seem to have frozen with NeXTSTEP and never
advanced with POSIX.

Remove dependency on libcrypto, include hash headers in a nicer way.

First attempt at building a new OpenNTPD portable project, based on the
latest OpenBSD source.