Get rid of NOCOLOR_NEXT.

Read environment variables that affect the current cw process before
reading definition file. Hence, environment variables set in the
definition file, like NOCOLOR, affect only processes launched by the
current cw process. Hence, there's no need for NOCOLOR_NEXT, as
NOCOLOR itself has the required effect.
cw.c: remove use of SIGUSR1: if there's no SIGCHLD, you have to interrupt cw
cw.c: establish signal handlers with sigaction for uniform semantics across systems
cw.c: try to read all output when child process quits