libunistd Upstream more additions/changes

This is the last of the changes from the Zeek project. It adds a few new methods that weren't there before, plus some cleanup that was required to build Zeek against libunistd.

The specific things I want to call out in this are:

6c8040003ab0c188ec33787fd349a8470ee9226a: The implementation of dirname causes a compiler warning because it's trying to shift a 32-bit value by 32 bits to store the high part of the offset. This could possibly just assign OffsetHigh to zero, but it's working for us as is.
58da4783e9c835175c91f6c21a7994fb611bab97: The implementation of read() causes the build to output a bunch of "inconsistent dll linkage" warnings when building the library outside of the Zeek project. The warnings go away with our build setup, but I haven't investigated what set of compiler flags make it do so. It does finish the build and link the libraries though.

Mar 16 '23 22:03 timwoj

I rebased this on the latest master and dropped the mdb commit that was there before, since that's removed from the repo now.

Mar 21 '23 20:03 timwoj

@robinrowe Any other comments on this or can it be merged?

Apr 12 '23 17:04 timwoj

Checking in on this one again. It'd be nice to swap over to using the main repo instead of our fork.

May 14 '23 17:05 timwoj

@robinrowe Checking in again

Jun 22 '23 16:06 timwoj

@robinrowe Was this ever going to get merged?

Sep 25 '23 16:09 timwoj