sys: mitigate possible strncpy string truncation

sys: mitigate possible strncpy string truncation

gcc 10 with -O2 reports following:

 In function ‘strncpy’,
     inlined from ‘rpc_sys_packagelist’ at /opt/devel/openwrt/c-projects/rpcd/sys.c:244:4:
 /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 128 equals destination size [-Werror=stringop-truncation]
   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In function ‘strncpy’,
     inlined from ‘rpc_sys_packagelist’ at /opt/devel/openwrt/c-projects/rpcd/sys.c:227:4:
 /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 128 equals destination size [-Werror=stringop-truncation]
   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since it is not possible to avoid truncation by strncpy, it is necessary
to make sure the result of strncpy is properly NUL-terminated and the
NUL must be inserted explicitly, after strncpy has returned.

References: #10442
Reported-by: Alexey Smirnov <s.alexey@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>