sysv init scripts: use symbolic names in trap command

Not really necessary(*), but checkbashisms complains about
"trap with signal numbers".

(*) Quoting man 1p trap:
   trap [action condition...]
   [...]
   The condition can be EXIT, 0 (equivalent to EXIT),
   or a signal specified using a symbolic name, without the SIG prefix
   [...]
   XSI-conformant systems also allow numeric signal numbers[...]

Only one file is affected by this commit, and it should be checked
whether it really needs to ignore SIGTERM/SIGHUP or if the trap commands
can simply be removed:

 package/proftpd/S50proftpd

Signed-off-by: André Erdmann <dywi@mailerd.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>