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/dpm/core.h

@@ -221,7 +221,7 @@ void dpm_close(dpm_ctx* ctx);
  * @param ctx The libdpm-core.so context
  * @param name The module name (its filename minus .so)
  * @return A module handle owned by the context, or NULL on failure
- *         with the precise reason retrievable via dpm_last_error()
+ *         with the precise reason retrievable via dpm_get_last_error()
  */
 DPM_PUBLIC_ABI_EXPORT
 dpm_module* dpm_require(dpm_ctx* ctx, const char* name);
@@ -356,7 +356,24 @@ void dpm_log(dpm_ctx* ctx, int level, const char* message);
  * @return The module directory path this context resolved
  */
 DPM_PUBLIC_ABI_EXPORT
-const char* dpm_module_path(dpm_ctx* ctx);
+const char* dpm_get_resolved_module_path(dpm_ctx* ctx);
+
+/**
+ * @brief Records a failure reason on the context
+ *
+ * A module reports why it failed by recording the reason here and
+ * returning nonzero, which carries detail its return code cannot.
+ *
+ * The context holds one reason at a time, so the most recent write is
+ * what dpm_get_last_error reports. Record the reason immediately before
+ * returning, so that later work does not replace it.
+ *
+ * @param ctx The libdpm-core.so context; NULL is a no-op
+ * @param msg The failure description, copied into the context; NULL is
+ *            a no-op
+ */
+DPM_PUBLIC_ABI_EXPORT
+void dpm_set_last_error(dpm_ctx* ctx, const char* msg);
 
 /**
  * @brief Returns the most recent failure recorded on the context
@@ -366,7 +383,7 @@ const char* dpm_module_path(dpm_ctx* ctx);
  *         NULL if none; overwritten by the next failing call
  */
 DPM_PUBLIC_ABI_EXPORT
-const char* dpm_last_error(dpm_ctx* ctx);
+const char* dpm_get_last_error(dpm_ctx* ctx);
 
 /* ------------------------------------------------------------------ */
 /* Module contract (implemented by modules, called by libdpm-core.so) */