LRN authored Apr 16, 2019 and Philip Withnall committed Apr 16, 2019

Commit 398008da added a W32-only code (from commit 7e0e251a)
to g_get_prgname() that makes this function never return NULL. This
is inconsistent with the other platforms. Revert the change, and add an
implementation for platform_get_argv0(), which is used by GOption when
g_get_prgname() == NULL.

The W32 platform_get_argv0() code is different from the one that was in
g_get_prgname(), because it should be getting argv0, not the name
of the executable that is being run (although most of the time they are
one and the same).

Adjust thest option-argv0 test to expect it to pass on W32.