Revert "fs_mgr: Drop the context mount option from non-selinux builds"

This reverts commit 806fc566fd4544cfcd3334aa1a992bab0f6d277c.

Revert "fs_mgr: Don't assume all fstab entries have options"

This reverts commit a73b2264dfd89d526054e4be9926be9f50cca791.
fs_mgr: Drop the context mount option from non-selinux builds

Transitioning devices may have the context option present in the
fstab, even if we're not doing selinux builds.
Since we don't set or even create the contexts on those builds,
this will make mount fail, and either the mountpoint ends up absent,
or boot fails entirely

Change-Id: Ic5cc7f5632cc3d62459dfaaf7719d662737e1641
Fix failure to build from source with modern _host_ glibc/gcc.

One should not define __USE_GNU directly, and instead _GNU_SOURCE
should be defined as early as possible, before first (indirect)
inclusion of features.h

Change-Id: I4d109d4fa0c3d266b873d2a0a66a52e1a4faa234
Signed-off-by: Dmitrijs Ledkovs <dmitrijs.ledkovs@canonical.com>