OCS_off_t should be a signed type

[jphickey]

Describe the bug
OCS_off_t is used for OSAL coverage testing to replace the C library off_t type as used in e.g. lseek()
This is being substituted with an unsigned type, but really should be a signed type.

To Reproduce
Suggested change in PR #592 exposes the mismatch:

https://github.com/klystron78/osal/blob/862f1322da247b1ae8a93a9e25e3f76bb5fa8d8e/src/os/portable/os-impl-posix-io.c#L106

Coverage test fails due to later comparison for (result < 0) which is impossible with unsigned type.

System observed on:
Ubuntu 20.04

Additional context
Using long instead fixes it.

Reporter Info
Joseph Hickey, Vantage Systems, Inc.