Merge branch 'jc/unsetenv-returns-an-int'

The compatibility implementation for unsetenv(3) were written to
mimic ancient, non-POSIX, variant seen in an old glibc; it has been
changed to return an integer to match the more modern era.

* jc/unsetenv-returns-an-int:
  unsetenv(3) returns int, not void

diff --cc git-compat-util.h
Simple merge