add --with-s6-log-timestamp flags, pass through write_common for module,split the write of environment file to env_compute function

Add man and install-man rules in Makefile,
add datarootdir and mandir in configure.

Add scdoc as dependency in INSTALL.

Ignore generated man pages in .gitignore.

implement --with-user-log,--with-packager-service,--with-{sys,user}-service,--with-{sys,user}-service-conf

service file is now into a directory, environment of the service is now by default at /etc/66/env/name_of_service