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/fuzz/fuzz-unit-file/directives.mount

@@ -5,6 +5,7 @@ AllowedMemoryNodes=
 AmbientCapabilities=
 AppArmorProfile=
 BPFProgram=
+ExecSearchPath=
 BindPaths=
 BindReadOnlyPaths=
 BlockIOAccounting=

test/fuzz/fuzz-unit-file/directives.service

@@ -31,6 +31,7 @@ AssertUser=
 AssertVirtualization=
 BPFProgram=
 Before=
+ExecSearchPath=
 BindTo=
 BindsTo=
 CollectMode=

test/fuzz/fuzz-unit-file/directives.socket

@@ -7,6 +7,7 @@ AmbientCapabilities=
 AppArmorProfile=
 BPFProgram=
 Backlog=
+ExecSearchPath=
 BindIPv6Only=
 BindPaths=
 BindReadOnlyPaths=

test/fuzz/fuzz-unit-file/directives.swap

@@ -5,6 +5,7 @@ AllowedMemoryNodes=
 AmbientCapabilities=
 AppArmorProfile=
 BPFProgram=
+ExecSearchPath=
 BindPaths=
 BindReadOnlyPaths=
 BlockIOAccounting=