pidof: allow to change a separator put between pids

I frequency use pidof command with strace system call tracer.
strace can trace MULTIPLE processes specified with "-p $PID"
arguments like:

	  strace -p 1 -p 1030 -p 3043

Sometimes I want to do as following

	  strace -p $(pidof httpd)

However, above command line doesn't work because -p option
is needed for specifying a pid. pidof uses a whitespace as
a separator. For passing the output to strace, the separator
should be replaced with ' -p '.

This maybe not a special to my use case.

This commit introduces -S option that allows a user to specify a
separator the one wants.

    $ ./pidof bash
    ./pidof bash
    24624 18790 12786 11898 11546 10766 7654 5095
    $ ./pidof -S ',' bash
    ./pidof -S ',' bash
    24624,18790,12786,11898,11546,10766,7654,5095
    $ ./pidof -S '-p ' bash
    ./pidof -S '-p ' bash
    24624-p 18790-p 12786-p 11898-p 11546-p 10766-p 7654-p 5095
    $ ./pidof -S ' -p ' bash
    ./pidof -S ' -p ' bash
    24624 -p 18790 -p 12786 -p 11898 -p 11546 -p 10766 -p 7654 -p 5095
    $ strace -p $(./pidof -S ' -p ' bash)
    strace -p $(./pidof -S ' -p ' bash)
    strace: Process 24624 attached
    strace: Process 18790 attached
    strace: Process 12786 attached
    ...

Signed-off-by: Masatake YAMATO <yamato@redhat.com>