Modules can report why they failed

A module had one channel back to its caller: the int returned from
dpm_module_execute, handed through by dpm_execute. Any detail behind
that number could only reach a log, so a caller wanting the reason had
to read output rather than ask for it.

dpm_set_last_error joins the exported API. A module records its reason
on the context handed to its entry point, which belongs to the caller,
and the caller reads it back with dpm_get_last_error.

The two accessors on the context now say what they do to it:
dpm_module_path becomes dpm_get_resolved_module_path, naming the value
it reports rather than the setting it came from, and dpm_last_error
becomes dpm_get_last_error, pairing with the setter.

Path normalization moves to sanitizers.cpp, which holds the conversions
that put a value written by a person into the single form the library
stores it in.

include/internal/context.hpp

@@ -89,5 +89,5 @@ namespace dpm_core {
      * @param ctx The libdpm-core.so context; NULL is a no-op
      * @param msg The failure description
      */
-    void set_error(dpm_ctx* ctx, const std::string& msg);
+    void set_last_error(dpm_ctx* ctx, const std::string& msg);
 } // namespace dpm_core