lsfd: print the masks specified in signalfds

lsfd: print the masks specified in signalfds

An example output:

    # ./lsfd -p "$(pidof systemd-journald)" -Q '(TYPE == "signalfd")'

    COMMAND             PID USER ASSOC MODE     TYPE       SOURCE MNTID INODE NAME
    systemd-journal 2382709 root   238  rw- signalfd anon_inodefs    15  1060 mask=USR1,USR2
    systemd-journal 2382709 root   239  rw- signalfd anon_inodefs    15  1060 mask=INT,TERM
    systemd-journal 2382709 root   240  rw- signalfd anon_inodefs    15  1060 mask=35

Using signum_to_signame is suggested by Karel Zak <kzak@redhat.com>.
Signed-off-by: Masatake YAMATO <yamato@redhat.com>