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.service

@@ -0,0 +1,4 @@
+[Service]
+ExecStart=ls_temp
+Type=oneshot
+ExecSearchPath=/tmp/test-exec_execsearchpath