posixlib errno.scala is incomplete

[LeeTibbert]

1. According to the POSIX 2018 specification for errno.h, the following ought to
   provide access to the errno variable:

   import scala.scalanative.posix.{errno => posixErrno, posixErrno.errno}
   

   [I will note and skip in passing the long standing flaw of the object having been named (lower case) errno,
   causing conflict with the enclosed variable. Fi!]

   Currently, in order to work in POSIX land, and use both errno and POSIX defined
   values for errno, one must import the variable errno from clib and then the
   values from posixib. The latter is comprehensible, the former is a case where
   we can and should do better.

    // Taken directly from unit-tests TimeTest.scala
    // Declare as:
    import scalanative.libc.{errno => libcErrno}
    import scala.scalanative.posix.errno.{EINVAL, EINTR}
   
   // Use as:
   assertEquals(
       s"clock_nanosleep failed with errno: ${libcErrno.errno}",
       EINTR,
       libcErrno.errno
     )
   

2. The list of possible values for errno should be checked against the specification.
   I know of no missing value but believe that validation would be worthwhile,
   if only bringing (my) peace of mind.

   Later: posixlib errno.scala was compared to the 2018 POSIX specification
   and the symbol EADDRNOTAVAIL was found to be missing in
   the former. See Issue posixlib errno.scala is missing symbol EADDRNOTAVAIL #2717.

[LeeTibbert]

I have posixlib errno variable working in my private area and hope to submit it after
PR #2718 is merged. That will keep the changes associated with the two different
concepts separated and ease PR submission & review.

The steps of the dance each take time...

[LeeTibbert]

resolved by merged PR #2721