Do not require the IANA PEN registry file

Previously, ipmitool would fail to run if the local copy of the IANA PEN
registry could not be parsed.

When the registry is not available the manufacturer will be "Unknown" but
ipmitool will otherwise function so should not be considered fatal.

Also, fix an issue with improperly handling the `oem_info_list_load`
return value. Previously, in `ipmi_oem_info_init`, if `oem_info_list_load`
returned a negative value due to the registry file not existing, an
improper count would cause `oem_info_init_from_list` to aallocate a list
that didn't encompass the full header/tail list.

  IANA PEN registry open failed: No such file or directory
    Allocating      3 entries
    [     1] 16777214 | A Debug Assisting Company, Ltd.
    [     0]  1048575 | Unspecified

Now, use a signed int and ensure a valid count of loaded OEMs is used.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>