[m4/ca_path_prog.m4] Added comment for AC_ARG_VAR
[m4/ca_path_python.m4] Removed duplication backslash to slash
[m4/ca_prog_cygpath_m.m4] if CYGPATH is not found set it with false
Revert "Added import gpg cygwin public signature to setup command"

This reverts commit 0cdb7ec592c4321f154dd98ce489bd0df1cc7b13.
Added Cygwin's public key to the gpg keyring on post install