[tacacs]: do not modify local user if there is no priviledge

check the uid before modify local user account.

when run sudo, the process the invoke nss_tacplus library
does not have priviledge to modify the user profile, and
will generate below error messages:

    user_rw@sonic:~$ sudo bash
    usermod: Permission denied.
    usermod: cannot lock /etc/passwd; try again later.
    usermod: Permission denied.
    usermod: cannot lock /etc/passwd; try again later.

Signed-off-by: Guohan Lu <lguohan@gmail.com>

src/tacacs/nss/patch/0008-do-not-create-or-modify-local-user-if-there-is-no-pr.patch

@@ -0,0 +1,32 @@
+From c59b775a7c9226954c5eea4ba05469879b41a60d Mon Sep 17 00:00:00 2001
+From: Guohan Lu <lguohan@gmail.com>
+Date: Sat, 6 Feb 2021 06:49:17 +0000
+Subject: [PATCH] do not create or modify local user if there is no privilege
+
+---
+ nss_tacplus.c | 9 +++++++--
+ 1 file changed, 7 insertions(+), 2 deletions(-)
+
+diff --git a/nss_tacplus.c b/nss_tacplus.c
+index fc9316f..cc6f0aa 100644
+--- a/nss_tacplus.c
++++ b/nss_tacplus.c
+@@ -629,8 +629,13 @@ static int lookup_user_pw(struct pwbuf *pb, int level)
+     if(0 != ret)
+         return ret;
+
+-    if(0 != create_or_modify_local_user(username, level, found))
+-        return -1;
++    if(0 == getuid()) {
++        if(0 != create_or_modify_local_user(username, level, found))
++            return -1;
++    } else {
++        if(debug)
++            syslog(LOG_DEBUG, "%d does not privilege to create or modify user %s", getuid(), username);
++    }
+
+     ret = lookup_pw_local(username, pb, &found);
+     if(0 == ret && !found) {
+-- 
+2.25.1
+

src/tacacs/nss/patch/series

@@ -5,3 +5,4 @@
 0005-libnss-Modify-parsing-of-IP-addr-and-port-number-str.patch
 0006-fix-compiling-warning-about-token-dereference.patch
 0007-Add-support-for-TACACS-source-address.patch
+0008-do-not-create-or-modify-local-user-if-there-is-no-pr.patch