gnetlist: Safer use of nettattrib-generated refdes.

Two-pronged attack to make gnetlist's use of autogenerated "POWER"
refdes less likely to cause a problem.

1) Use a longer, more specific prefix string.  This will make it much
   much less likely that a collision with a user net name will occur.

2) When testing for autogenerated refdes, test for the exact prefix
   using strncmp() instead of just checking that it appears somewhere
   in the string.

Affects-bug: lp-1226246
gnetlist: Error on clashes with netattrib-generated refdes.

Exit with an error if a refdes is found in input files that conflicts
with autogenerated netattrib-related refdes.

Closes-bug: lp-1226246