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
(imported from commit
c58247b598517b875d8cb2b80eb812e92ee9c87f)