Add getconf(1)

The logic is simple, it's just a pain in the ass to fill the
data-structures.
Some lines had to be commented out, as glibc/musl apparently
have not fully implemented the mandatory variables for the
2013 corrigendum of POSIX 2008.

Also added a manpage and the necessary entries in README.

I also removed it from the TODO.

README

@@ -36,6 +36,7 @@ The following tools are implemented:
 =*|o false       .
 =    find        .
 #*|o fold        .
+=* o getconf     (-v)
 =* o grep        .
 =*|o head        .
 =*|x hostname    .