statmount: clean up unescaped option handling

Move common code from opt_array/opt_sec_array to helper.  This helper
does more than just unescape options, so rename to
statmount_opt_process().

Handle corner case of just a single character in options.

Rename some local variables to better describe their function.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20241120142732.55210-1-mszeredi@redhat.com


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>