rc: Honor ${name}_env when a custom *_cmd is defined (e.g., start_cmd)

A user may set ${name}_env variable in rc.conf(5) in order to set additional
environment variables for a service command.  Unfortunately, at the moment
this variable is only honored when the command is specified via the command
variable. Those additional environment variables coming from ${name}_env
are never set if the service is started via the ${rc_arg}_cmd variable (for
example start_cmd).

PR:		239692
Reviewed by:	bcr, jilles
Approved by:	src (jilles)
Differential Revision:	https://reviews.freebsd.org/D21228

libexec/rc/rc.subr

@@ -1036,6 +1036,9 @@ run_rc_command()
 		     _postcmd=\$${rc_arg}_postcmd
 
 		if [ -n "$_cmd" ]; then
+			if [ -n "$_env" ]; then
+				eval "export -- $_env"
+			fi
 			_run_rc_precmd || return 1
 			_run_rc_doit "$_cmd $rc_extra_args" || return 1
 			_run_rc_postcmd

share/man/man8/rc.subr.8

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 5, 2019
+.Dd September 5, 2019
 .Dt RC.SUBR 8
 .Os
 .Sh NAME
@@ -559,9 +559,19 @@ is mounted.
 A list of environment variables to run
 .Va command
 with.
-This will be passed as arguments to the
+Those variables will be passed as arguments to the
 .Xr env 1
-utility.
+utility unless
+.Ar argument Ns Va _cmd
+is defined.
+In that case the contents of
+.Va ${name}_env
+will be exported via the
+.Xr export 1
+builtin of
+.Xr sh 1 ,
+which puts some limitations on the names of variables
+(e.g., a variable name may not start with a digit).
 .It Va ${name}_env_file
 A file to source for environmental variables to run
 .Va command