Instead of hardcoding /proc/self/cmdline use for __linux__ only,
do a configure-time test for it.

Specifically, this enables /proc/self/cmdline use on Cygwin.

The configure-time test is very primitive (just tests that the
file exists and that it's possible to read more than one byte from it),
relying on the testsuite for more extensive checks.

The test in the testsuite is modified to always run, even on platforms
where it isn't supposed to pass. If it fails there, the testing framework
skips it. If the test unexpectedly passes, that is reported too.