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-environment-path-set.service

@@ -0,0 +1,5 @@
+[Service]
+ExecStart=/bin/sh -x -c 'test "$$PATH" = "/usr" && test "$$VAR1" = word3 && test "$$VAR2" = "\\$$word 5 6"'
+Type=oneshot
+ExecSearchPath=/tmp:/bin
+Environment="PATH=/usr" VAR1=word3 "VAR2=$word 5 6"