Use a consistent errno save/restore pattern before running strtoul

• Save errno
• Set errno to 0
• Call strtoul
• Test errno (optional, but many calls to strtoul did this afterwards)

Some of the code was setting errno = 0 after calling strtoul, not setting
errno = 0, or setting errno to saved_errno after the call, but before the
test. These all have unwanted behavioral side-effects, depending on the
initial value of errno and whether or not the input to strtoul was correct
or incorrect.

MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division