Use correct product name in `User-Agent` value

src/BizHawk.Common/Extensions/StringExtensions.cs

@@ -43,6 +43,10 @@ namespace BizHawk.Common.StringExtensions
 		public static bool In(this string str, params string[] options) =>
 			options.Any(opt => string.Equals(opt, str, StringComparison.OrdinalIgnoreCase));
 
+		/// <returns>a copy of <paramref name="raw"/> with all characters outside <c>[0-9A-Za-z]</c> removed</returns>
+		public static string OnlyAlphanumeric(this string raw)
+			=> string.Concat(raw.Where(static c => c is (>= '0' and <= '9') or (>= 'A' and <= 'Z') or (>= 'a' and <= 'z')));
+
 		/// <returns>
 		/// <paramref name="str"/> with the first char removed, or
 		/// the original <paramref name="str"/> if the first char of <paramref name="str"/> is not <paramref name="prefix"/>

src/BizHawk.Common/VersionInfo.cs

@@ -39,7 +39,9 @@ namespace BizHawk.Common
 					CustomBuildString = lines[0];
 				}
 			}
-			UserAgentEscaped = $"BizHawk/{MainVersion}{(DeveloperBuild ? "-dev" : string.Empty)}";
+			UserAgentEscaped = $"{
+				(string.IsNullOrWhiteSpace(CustomBuildString) ? "EmuHawk" : CustomBuildString!.OnlyAlphanumeric())
+			}/{MainVersion}{(DeveloperBuild ? "-dev" : string.Empty)}";
 		}
 
 		public static (string Label, string TargetURI) GetGitCommitLink()