Provide a tree by default and handle it name at compilation time with the --with-default-tree-name flag

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.