win32: more efficient tests for executables

file_is_executable() calls access(2) and stat(2); in this case our
WIN32 implementation also calls stat(2).  Avoid this unnecessary
duplication by copying the required test from the WIN32 access(2).

In find_executable() it's possible to avoid calling file_is_executable()
if add_win32_extension() returns TRUE as the latter will already have
verified that the file is executable.

Replace a call to file_is_executable() in the tab completion code
with the equivalent tests, avoiding (up to) two calls to stat(2).

These changes don't affect the size of the binary and should be
faster.

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