misc: Introduce internal function ssh_dir_writeable()

The introduced internal function checks if the provided path is for an
existing directory which is accessible for writing.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 7857cd1aa5)

include/libssh/misc.h

@@ -26,6 +26,7 @@
 char *ssh_get_user_home_dir(void);
 char *ssh_get_local_username(void);
 int ssh_file_readaccess_ok(const char *file);
+int ssh_dir_writeable(const char *path);
 
 char *ssh_path_expand_tilde(const char *d);
 char *ssh_path_expand_escape(ssh_session session, const char *s);