avoid using strok - eliminates use of hidden global variable

function                                             old     new   delta
udhcp_str2optset                                     616     650     +34
setpriv_main                                         950     975     +25
switch_root_main                                     688     706     +18
parse                                                958     970     +12
getopt_main                                          622     628      +6
parse_resolvconf                                     302     306      +4
mpstat_main                                         1139    1142      +3
static.p                                               4       -      -4
cdcmd                                                717     702     -15
strtok                                               148       -    -148
------------------------------------------------------------------------------
(add/remove: 0/3 grow/shrink: 7/1 up/down: 102/-167)          Total: -65 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/update_passwd.c b/libbb/update_passwd.c
index c605c4c..7b67f30 100644
--- a/libbb/update_passwd.c
+++ b/libbb/update_passwd.c
@@ -18,17 +18,20 @@
 #if ENABLE_SELINUX
 static void check_selinux_update_passwd(const char *username)
 {
-	security_context_t context;
-	char *seuser;
+	security_context_t seuser;
+	char *p;
 
 	if (getuid() != (uid_t)0 || is_selinux_enabled() == 0)
 		return;  /* No need to check */
 
-	if (getprevcon_raw(&context) < 0)
+	if (getprevcon_raw(&seuser) < 0)
 		bb_simple_perror_msg_and_die("getprevcon failed");
-	seuser = strtok(context, ":");
-	if (!seuser)
-		bb_error_msg_and_die("invalid context '%s'", context);
+
+	p = strchr(seuser, ':');
+	if (!p)
+		bb_error_msg_and_die("invalid context '%s'", seuser);
+	*p = '\0';
+
 	if (strcmp(seuser, username) != 0) {
 		security_class_t tclass;
 		access_vector_t av;