core: Add ExecSearchPath parameter to specify the directory relative to which binaries executed by Exec*= should be found

Currently there does not exist a way to specify a path relative to which
all binaries executed by Exec should be found. The only way is to
specify the absolute path.

This change implements the functionality to specify a path relative to which
binaries executed by Exec*= can be found.

Closes #6308

test/test-execute/exec-execsearchpath-unit-specifier.service

@@ -0,0 +1,7 @@
+[Unit]
+Description=Test for specifiers with exec search path
+
+[Service]
+Type=oneshot
+ExecSearchPath=/tmp:/bin:/usr/bin:%V
+ExecStart=/bin/sh -x -c 'test %V = /var/tmp && test "$$PATH" = "/tmp:/bin:/usr/bin:/var/tmp"'