gcc8.1: fix strncpy bounds warnings

gcc8.1: fix strncpy bounds warnings

These warnings are coming on default Fedora 28 build and probably others using gcc 8.1

../shared.c: In function ‘expand_macro’:
../shared.c:483:3: warning: ‘strncpy’ specified bound depends on the length of the source argument [-Wstringop-overflow=]
   strncpy(name, value, len);
   ^~~~~~~~~~~~~~~~~~~~~~~~~
../shared.c:480:9: note: length computed here
   len = strlen(value);
         ^~~~~~~~~~~~~

strncpy with a computed length via strlen is usually
not the right thing.

Signed-off-by: Andy Green <andy@warmcat.com>