xfail box-related debug-info tests r=snapshot
ignore inherited environment test under valgrind r=snapshot
_Actually_ xfail debuginfo tests
libc: support functions from sys/mman.h

Because its part of POSIX. Values are taken from header FreeBSD, linux
and OSX header files.
libc: add POSIX-compatible sysconf consts

Because its part of POSIX. Values are taken from header FreeBSD, linux
and OSX header files.