getprogname: port to systems with __argv (mingw, msvc)

* lib/getprogname.c (getprogname): Include "dirname.h" and use
last_component: more general than open coding it with hard-coded "/".
* lib/getprogname.h (getprogname): Prefer "char const *" consistently.
* modules/getprogname (Depends-on): Add dirname-lgpl.
(configure.ac): Check for __argv in <stdlib.h>.
* modules/getprogname-tests: New file.
* tests/test-getprogname.c: New file.
Suggested by Gisle Vanem in
https://lists.gnu.org/archive/html/bug-gnulib/2016-09/msg00014.html

modules/getprogname-tests

@@ -0,0 +1,13 @@
+Files:
+tests/test-getprogname.c
+
+Depends-on:
+assert-h
+string
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-getprogname
+check_PROGRAMS += test-getprogname
+test_getprogname_LDADD = $(LDADD)