Process control:

- Implemented priority control on Unix platform
- Refactored RunnableThreadUnix implementation
- Bump resource limits for process scheduling priority (nice) if possible

#rb LouisPhilippe.Seguin, Brandon.Schaefer
[FYI] anrew.ladenberger

[CL 22087348 by anton dunchev in ue5-main branch]

Engine/Source/Programs/UnrealBuildTool/Configuration/TargetRules.cs

@@ -1599,6 +1599,13 @@ namespace UnrealBuildTool
 		/// </summary>
 		public bool bEnableCppCoroutinesForEvaluation = false;
 
+		/// <summary>
+		/// Whether to enable engine's ability to set process priority on runtime.
+		/// This option requires some environment setup on Linux, that's why it's disabled by default.
+		/// Project has to opt-in this feature as it has to guarantee correct setup.
+		/// </summary>
+		public bool bEnableProcessPriorityControl = false;
+
 		/// <summary>
 		/// If true, then enable memory profiling in the build (defines USE_MALLOC_PROFILER=1 and forces bOmitFramePointers=false).
 		/// </summary>
@@ -3222,6 +3229,11 @@ namespace UnrealBuildTool
 			get { return Inner.bEnableCppCoroutinesForEvaluation; }
 		}
 
+		public bool bEnableProcessPriorityControl
+		{
+			get { return Inner.bEnableProcessPriorityControl; }
+		}
+
 		public bool bUseMallocProfiler
 		{
 			get { return Inner.bUseMallocProfiler; }