Catch more strtol() failures when using KEYRINGs

When parsing what should be a UID while resolving a KEYRING ccache
name, don't just depend on strtol() to set errno when the residual
that we pass to it can't be parsed as a number.  In addition to
checking errno, pass in and check the value of an "endptr".

[ghudson@mit.edu: simplified slightly]

ticket: 7764 (new)
target_version: 1.12
tags: pullup