for compiled executables, try src(1).

in this particular case, however, the services are implemented by shell scripts.
see listen(8) for details, but they are found in /rc/bin/service and /rc/bin/service.auth,
in files named using an obvious naming scheme (eg, /rc/bin/service/tcp7).

	term% cat /bin/service/tcp7
	#!/bin/rc
	/bin/cat

so much for that one.  others such as tcp25 invoke specialised commands
with appropriate argument shuffling:

	term% cat /bin/service/tcp25
	#!/bin/rc
	#smtp serv net incalldir user

	user=`{cat /dev/user}
	exec upas/smtpd -n $3

and then you can do
	src /bin/upas/smtpd
to pop the source of the command into an acme frame, or use
src -n just to print the name:

	term% src -n /bin/upas/smtpd
	/sys/src/cmd/upas/smtp/smtpd.c:71