rcutil: rename rcpath to rccomponents (API)

rcutil: extract rc directory listing logic The logic of listing a ".rc" directory is duplicated in two functions, extract it to a single function to make the code cleaner.

rcutil: split osrcpath to return default.d paths (API) After this change, there are 3 rcpath functions: - defaultrcpath - systemrcpath - userrcpath This will allow us to insert another config layer in the middle.

rcutil: move scmutil.*rcpath to rcutil (API) As discussed at [1], the logic around "actual config"s seem to be non-trivial enough that it's worth a new module. This patch creates the module and move "scmutil.*rcpath" functions there as the first step. More methods will be moved to the module in the future. The module is different from config.py because the latter only cares about data structure and parsing, and does not care about special case, or system config paths, or environment variables. [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/095503.html

statfs: make getfstype() raise OSError It's better for getfstype() function to not suppress an error. Callers can handle it as necessary. Now "hg debugfsinfo" will report OSError.

statfs: rename pygetfstype to getfstype There's no ambiguity now.

statfs: refactor inner function as a mapping from statfs to string The major difference between BSD and Linux is how to get a fstype string. Let's split the longest part of getfstype() as a pure function.

statfs: simplify handling of return value Py_BuildValue() can translate NULL pointer to None.