Limit usage of extern to config.h globals

Prefer passing arguments to declaring external global variables.  The
only remaining usage of extern is for config.h variables which are
needed in st.c instead of x.c (where it is now included).

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

config.def.h

@@ -16,7 +16,7 @@ static int borderpx = 2;
  * 4: value of shell in /etc/passwd
  * 5: value of shell in config.h
  */
-char *shell = "/bin/sh";
+static char *shell = "/bin/sh";
 char *utmp = NULL;
 char *stty_args = "stty raw pass8 nl -echo -iexten -cstopb 38400";