libmount: separate fstype from mounttype, add config file support - #4583

Separate the on-disk filesystem type (fstype, e.g. "ntfs") from the
kernel FS driver name (mounttype, e.g. "ntfs3"). Previously,
ul_fstype_to_mounttype() completely overwrote the fstype with the
driver name, causing helper lookup to search for /sbin/mount.<mounttype>
instead of /sbin/mount.<fstype>.

Now fstype and mounttype are stored separately in struct libmnt_fs:
 - fstype is used for helper search (/sbin/mount.<fstype>) and
   user-facing display
 - mounttype is used for mount(2) and fsopen() syscalls

The mounttype mapping only applies to auto-detected types. When the
user explicitly specifies a type via -t or fstab, it is used as-is
for both helper search and the syscall -- no remapping.

By default, no fstype-to-mounttype mapping is defined. Use
--with-ntfs-mounttype=<type> (or -Dntfs-mounttype=<type>) to define
a compile-time mapping for NTFS.

Addresses: util-linux#3618
Addresses: util-linux#4307

Signed-off-by: Karel Zak <kzak@redhat.com>

Add /etc/mount/fs.d/<fstype>.conf config file support to override the
kernel driver name (mounttype) for auto-detected filesystem types.

The config file uses simple "key = value" format:

  # /etc/mount/fs.d/ntfs.conf
  mounttype = ntfs3

The config file lookup uses ul_configs_file_list() with standard
three-tier priority (admin > runtime > vendor):

  /etc/mount/fs.d/<fstype>.conf
  /run/mount/fs.d/<fstype>.conf
  /usr/lib/mount/fs.d/<fstype>.conf

Priority for mounttype resolution:
  config file > hardcoded ul_fstype_to_mounttype() > no remapping

Config values are cached on the mount context so each fs.d/<name>
is parsed only once per context lifetime.

Addresses: util-linux#3618
Addresses: util-linux#4307

Signed-off-by: Karel Zak <kzak@redhat.com>

Create a config file in /run/mount/fs.d/ with a fake mounttype,
attempt to mount an ext2 device, and verify that mount(8) reports
the configured (non-existent) kernel driver name in the error.

Skip on platforms without fsopen() to avoid unstable errno
differences with legacy mount(2).

Signed-off-by: Karel Zak <kzak@redhat.com>