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/finit.c

@@ -729,10 +729,7 @@ int main(int argc, char *argv[])
 	/*
 	 * Set PATH, SHELL, and PWD early to something sane
 	 */
-	setenv("PATH", _PATH_STDPATH, 1);
-	setenv("SHELL", _PATH_BSHELL, 1);
-	setenv("LOGNAME", "root", 1);
-	setenv("USER", "root", 1);
+	conf_reset_env();
 
 	if (chdir("/"))
 		_pe("Failed cd /");