also return nil for the empty env vars in the generic implementation in GetEnvPChar

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24733 3ad0048d-3df7-0310-abae-a5850022a9f2
don't allow an env var with invalid characters, also fixed a signedness warning

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24734 3ad0048d-3df7-0310-abae-a5850022a9f2
* always use 16-bit pushes for pushing/popping the segment registers on i8086 in Tcgx86.g_concatcopy

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24735 3ad0048d-3df7-0310-abae-a5850022a9f2