Deduplicate PATH entries that are symlinks resolving to other PATH entries

[DimCitus]

In recent debian releases /bin is a symlink to /usr/bin, which means we're
going to find pg_config twice on that system. But it's twice the same entry,
and we can deduplicate and realize we only have one entry actually.

Fixes #545

[JelteF]

There's this warning in travis:

/file_utils.Po -o file_utils.o file_utils.c

In file included from /usr/include/stdlib.h:1020:0,

                 from /usr/include/postgresql/10/server/c.h:82,

                 from /usr/include/postgresql/10/server/postgres_fe.h:25,

                 from file_utils.c:18:

In function ‘realpath’,

    inlined from ‘search_path_deduplicate_symlinks’ at file_utils.c:620:7:

/usr/include/x86_64-linux-gnu/bits/stdlib.h:43:9: warning: call to ‘__realpath_chk_warn’ declared with attribute warning: second argument of realpath must be either NULL or at least PATH_MAX bytes long buffer

  return __realpath_chk_warn (__name, __resolved, __bos (__resolved));

         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Also, all of the tests are failing (possibly because of the same thing).

[DimCitus]

There's this warning in travis:

Ah yeah, realpath requires PATH_MAX. Fixed now, thanks for pointing it to me!