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-environmentfile-set.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Test for ExecSearchPath with EnvironmentFile where EnvironmentFile sets PATH
+
+[Service]
+ExecStart=/bin/sh -x -c 'test "$$VAR1" = "word1 word2" && test "$$VAR2" = word3 && test "$$VAR3" = "\\$$word 5 6" && test "$$VAR4" = "new\nline" && test "$$VAR5" = passwordwithbackslashes && test "$$PATH" = /usr'
+Type=oneshot
+EnvironmentFile=/tmp/test-exec_execsearchpath_environmentfile-set.conf
+ExecSearchPath=/tmp:/bin