cal: Add the `-y` option to show the current year

Without `-y`, to show the current full year you'd have to specify which
one: `cal 2023`. Adding `-y` makes it possible to see the full current
year without remembering what year we are in.

This option is also stolen from FreeBSD :^)

Additionally, validate args: prevent passing both -3 and -y at the
same time. Passing both `--three-month-mode` and `--year` to `cal`
doesn't make sense. You'd either want the one or the other.

Base/usr/share/man/man1/cal.md

@@ -5,7 +5,7 @@ cal - Display a calendar
 ## Synopsis
 
 ```**sh
-$ cal [--starting-day weekday] [--three-month-view] [[month] year]
+$ cal [--starting-day weekday] [--three-month-view] [--year] [[month] year]
 ```
 
 ## Description
@@ -24,6 +24,7 @@ Days, months and years are specified with numbers. Week starts at Sunday.
 
 * `-s`, `--starting-day`: Specify which day should start the week. Accepts either short or long weekday names or indexes (0 being Sunday).
 * `-3`, `--three-month-view`: Display the previous, current, and next months side-by-side.
+* `-y`, `--year`: Display an entire year by laying out months on a grid. If no year number is specified, the current year is used as a default.
 
 ## Examples
 

Userland/Utilities/cal.cpp

@@ -169,6 +169,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     int year = 0;
     StringView week_start_day_name {};
     bool three_month_mode = false;
+    bool year_mode = false;
 
     Core::ArgsParser args_parser;
     args_parser.set_general_help("Display a nice overview of a month or year, defaulting to the current month.");
@@ -176,9 +177,15 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     args_parser.add_positional_argument(month, "Month", "month", Core::ArgsParser::Required::No);
     args_parser.add_positional_argument(year, "Year", "year", Core::ArgsParser::Required::No);
     args_parser.add_option(week_start_day_name, "Day that starts the week", "starting-day", 's', "day");
+    args_parser.add_option(year_mode, "Show the whole year at once", "year", 'y');
     args_parser.add_option(three_month_mode, "Show the previous and next month beside the current one", "three-month-view", '3');
     args_parser.parse(arguments);
 
+    if (three_month_mode && year_mode) {
+        warnln("cal: Cannot specify both --year and --three-month-mode at the same time");
+        return 1;
+    }
+
     time_t now = time(nullptr);
     auto* tm = localtime(&now);
     current_year = tm->tm_year + 1900;
@@ -191,7 +198,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         month = 0;
     }
 
-    bool year_mode = !month && year;
+    if (!month && year)
+        year_mode = true;
 
     int week_start_day;
     if (week_start_day_name.is_empty())