procd: update procd.sh to disallow signal-numbers, enforce signal-names

A given signal-name is now converted to the corresonding number. In general
it's good style to use names (readability) and it's more portable: signal
numbers can be architecture-dependent, so we are more safe giving names.

A real world example is signal 10, which is BUS on ramips and USR1 on PPC.

All users of 'procd_send_signal' must change their code to reflect this.

Signed-off-by: Bastian Bittorf <bb@npl.de>