Silence warning in select.cc

Silence warning in select.cc

With GCC 9.3, at least, compiling select.cc spits out a warning about
an implausible bound being passed to strncmp. This comes from inlining
IdString::compare(): it turns out that passing std::string::npos as a
bound to strncmp triggers it.

This patch replaces the compare call with a memcmp with the same
effect. The repeated calls to IdString::c_str are slightly
inefficient, but I'll address that in a follow-up commit.