Improve error reporting in fsu_mount()

Currently when fsu_mount() fails for some reason no feedback is provided
to the user:

    $ ./fsu_ls /tmp/nonexistent.udf
    usage: lt-fsu_ls [-o mnt_args] [-s specopts] [-t fstype] [-f] fsdevice [-AaBbCcdFfghikLlmnopqRrSsTtuWwx1] [file ...]

This commit improves the error reporting to be more helpful:

    $ ./fsu_ls /tmp/nonexistent.udf
    lt-fsu_ls: /tmp/nonexistent.udf: No such file or directory
    usage: lt-fsu_ls [-o mnt_args] [-s specopts] [-t fstype] [-f] fsdevice [-AaBbCcdFfghikLlmnopqRrSsTtuWwx1] [file ...]
    $ ./fsu_ls /tmp/bogus
    efs: invalid superblock: sb_magic invalid
    lt-fsu_ls: /tmp/bogus: Invalid or unknown filesystem type, retry with -v for details
    usage: lt-fsu_ls [-o mnt_args] [-s specopts] [-t fstype] [-f] fsdevice [-AaBbCcdFfghikLlmnopqRrSsTtuWwx1] [file ...]
    $ ./fsu_ls /dev/zero
    lt-fsu_ls: /dev/zero: Not a regular file or block device
    usage: lt-fsu_ls [-o mnt_args] [-s specopts] [-t fstype] [-f] fsdevice [-AaBbCcdFfghikLlmnopqRrSsTtuWwx1] [file ...]
fsu_mount(): Fix rump_pub_etfs_register() error handling

The rump_pub_*() functions return 0 on success and a NetBSD errno on
failure. Handle this correctly.
Merge pull request #16 from mato/wip-fsu-mount

Improve error reporting in fsu_mount()