A PAM transaction needs to be ended in order to release the associated
resources, however this can't be sadly automated as the go finalizers
run in goroutines and this could cause problems to modules that we load.
In fact a module code may be called back during pam_end (to cleanup data
for example) and the module code could not be thread safe.
So let's make this more manual, but safer.
The transaction status is still preserved in the transaction so end will
be automatically called with the last-known status.
Closes: #14
While transaction does implement error, it's not a valid error
implementer because it may have bogous values since it's not thread-safe
and so we may read the result of Error() when it's into an invalid state
As per this never return it as an error, while always return the Status
unless when not available, where we still return pam.Error.
If the transaction fails during start, there's no way to get the error
detail in a programmatic way, so let's wrap the pam.Error to allow more
per-type checks.
