[SV 63219] Support an "unload" function for loaded objects

[SV 63219] Support an "unload" function for loaded objects

If a loaded object defines a symbol <object>_gmk_unload, assume it's
a function and invoke it whenever the loaded object is unloaded.
Original implementation by Dmitry Goncharov <dgoncharov@users.sf.net>

* NEWS: Announce the change.
* doc/make.texi: Describe the behavior.
* src/gnumake.h: Add information to the comments.
* src/makeint.h (unload_all): Declare a new function.
* src/main.c (die): Invoke unload_all().
* src/load.c (unload_func_t): Declare a new type for unload.
(struct load_list): Remember the unload symbol if it exists.
(load_object): Move the parsing of the object name from load_file.
Check for the _gmk_unload symbol and if found, remember it.
(load_file): Allow load_object to do object filename parsing.
(unload_file): Remove the load_list entry when unloading the object.
(unload_all): Unload all the loaded objects.
* tests/scripts/features/loadapi: Test the unload function.