Merge branch 'bw/realpath-wo-chdir'

Merge branch 'bw/realpath-wo-chdir'

The implementation of "real_path()" was to go there with chdir(2)
and call getcwd(3), but this obviously wouldn't be usable in a
threaded environment.  Rewrite it to manually resolve relative
paths including symbolic links in path components.

* bw/realpath-wo-chdir:
  real_path: set errno when max number of symlinks is exceeded
  real_path: prevent redefinition of MAXSYMLINKS