Shell: Quick fix to not freeze for multiple seconds

When sync is writing to disk, it reserves whole disk to itself.
This commit makes Shell to read username only once from getpwuid().
We used to get username every time prompt was printed.

userspace/Shell/main.cpp

@@ -701,11 +701,17 @@ BAN::String get_prompt()
 			}
 			case 'u':
 			{
-				auto* passwd = getpwuid(geteuid());
-				if (passwd == nullptr)
-					break;
-				MUST(prompt.append(passwd->pw_name));
-				endpwent();
+				static char* username = nullptr;
+				if (username == nullptr)
+				{
+					auto* passwd = getpwuid(geteuid());
+					if (passwd == nullptr)
+						break;
+					username = new char[strlen(passwd->pw_name) + 1];
+					strcpy(username, passwd->pw_name);
+					endpwent();
+				}
+				MUST(prompt.append(username));
 				break;
 			}
 			case 'h':