Disable is_dir_fast() on Windows

Windows reports that "/path/to/file/." is directory.
Fix misc/strchar2str.c test on cygwin
Fix misc/cmdline_completion test on cygwin
Disable mkdir permission test on cygwin

Permissions doesn't affect system calls, so tests fail otherwise.
Don't use `find` to discover tests executables
Fix unix cygwin environment detection for tests
Return non i/o implementation of some ops

They should be executed when 'syscalls' isn't set.
Warn if GTK discovery in configure has failed
Wording updates for 'vifminfo' description
Merge branch 'various-unrelated-updates'

Various fixes for cygwin, some for Windows, for Unix and for all
systems.