doc: refer to README.md to find out more information about tests

Reason for this change is because 'make test' command does not work.

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
process: adjust quote_cmd_arg for empty arguments

adjust quote_cmd_arg so that empty command line
arguments are double quoted instead of skipped
win: minor coding style cleanup
win: open_named_pipe const argument
unix, windows: uv_dlerror const argument