uname(3): Add env override capability

uname(3): Add env override capability

The uname(1) program has honored UNAME_[x] overrides since DragonFly's
beginning, but libc's uname never did.  Make it recognize the following
environment variables:
  UNAME_m
  UNAME_r
  UNAME_s
  UNAME_v
These variables cause uname() to return the specified values instead of
what was built into libc.  This capability, which has been in FreeBSD for
more than a decade, is required for package builders that need to define
the jail environment different from the host (prime example: Python)