Add support for setting environment variables from finit.conf

Finit can now track and set environment variables from finit.conf.  The
syntax is pretty much what you'd expect:

    foo=bar
    baz="qux"

On reload of .conf files, all tracked environment variables are cleared
so if `foo=bar` is removed from finit.conf, or any finit.d/*.conf file,
it will no longer be used by Finit or any new (!) started run/tasks or
services.

The only variables reset to sane defaults on .conf reload are:

    PATH=_PATH_STDPATH
    SHELL=_PATH_BSHELL
    LOGNAME=root
    USER=root

It is entirely possible to override these as well from the .conf files,
but be careful.  Changing SHELL changes the behavior of system() and a
lot of other commands as well.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/conf.h

@@ -42,6 +42,7 @@ int  conf_any_change      (void);
 int  conf_changed         (char *file);
 int  conf_monitor         (void);
 
+void conf_reset_env       (void);
 void conf_parse_cmdline   (int argc, char *argv[]);
 int  conf_parse_runlevels (char *runlevels);
 void conf_parse_cond      (svc_t *svc, char *cond);