a first stab at a 'trace' option
the trace option should show all options

a large 'input' option will probably display badly
(need some quoting?)
POD fix

links to other modules using L<>
remove C<> around self-references to the module name
make the documentation easier to link to

the =head2 titles only contain the method names
and the details are in a SYNOPSIS-like section below
link to System::Command::Reaper when talking about zombies
Merge branch 'trace'

Conflicts:
	lib/System/Command.pm
simpler version of the trace option

behaviour copied from DBI_TRACE
open the trace file before changing directory

this should avoid issues (like "where is my trace file?")
when using relative names for the trace file
also trace failures to spawn the command
dump something readable when the option is a reference
when SYSTEM_COMMAND_TRACE is given, open the file at startup

this makes the trace more tolerant to chdir() calls during the
life of the program
trace will quote command parameters containing spaces
improved trace example
only trace failure if trace is requested
always quote options

this allows to properly differentiate between undef and "undef"
more trace documentation
document what is traced when spawn fails
follow the naming conventions for private subs
do a full trace when failing

and factor out the trace code in a coderef, now that it's called from several places
change the trace format