Simplify SETPROP macro

Simplify SETPROP macro

Get rid of xargs by using a subshell instead.
Use two replacement commands in sed because back-references in BRE are
too differently implemented in different libc.
Rename p, q, to reflect what they're used for.
Reorder sh positional arguments.
Set $0 to a command name and shift other positional parameters to
clarify the "sh -c" command.