libc: support functions from sys/mman.h

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

Because its part of POSIX. Values are taken from FreeBSD, linux
and OSX header files.
libc: (u)int => c_(u)int for consts
libc: fix return type of mmap, MAP_FAILED: *c_void

Its totally uncomfortable to work with `mmap()` if it doesn't return
mutable pointer. Also return value should be checked against
`MAP_FAILED`, so it should have `*c_void` type too.
auto merge of #7257 : indutny/rust/feature/libc-mman, r=cmr

Add functions and constants required to be able to use `mmap` and friends.