roadmap: describe glibc commands.

Some glibc commands are irrelevant because they're for functionality
that is excluded from musl (mtrace, rpc*, localedef, iconvconfig, nscd).
getconf and catchsegv look like candidates for the development toolchain;
locale and iconv were already triaged.
getent is pretty lame, but it and the timezone stuff (tzselect zic
zdump) are the only new possibly interesting commands.
On further analysis, none of the glibc commands are interesting.