LibC: Add timegm()

timegm() is like mktime() in that it converts a struct tm to
a timestamp, but it treats the struct tm as UTC instead of as
local time.

timegm() is nonstandard, but availabe in both Linux and BSD,
and it's a useful function to have.

Libraries/LibC/time.cpp

@@ -93,7 +93,7 @@ static void time_to_tm(struct tm* tm, time_t t)
     tm->tm_mday += days;
 }
 
-time_t mktime(struct tm* tm)
+static time_t tm_to_time(struct tm* tm, long timezone_adjust)
 {
     int days = 0;
     int seconds = tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec;
@@ -107,7 +107,12 @@ time_t mktime(struct tm* tm)
         ++tm->tm_yday;
 
     days += tm->tm_yday;
-    return days * __seconds_per_day + seconds + timezone;
+    return days * __seconds_per_day + seconds + timezone_adjust;
+}
+
+time_t mktime(struct tm* tm)
+{
+    return tm_to_time(tm, timezone);
 }
 
 struct tm* localtime(const time_t* t)
@@ -124,6 +129,11 @@ struct tm* localtime_r(const time_t* t, struct tm* tm)
     return tm;
 }
 
+time_t timegm(struct tm* tm)
+{
+    return tm_to_time(tm, 0);
+}
+
 struct tm* gmtime(const time_t* t)
 {
     static struct tm tm_buf;

Libraries/LibC/time.h

@@ -54,6 +54,7 @@ typedef int64_t time_t;
 struct tm* localtime(const time_t*);
 struct tm* gmtime(const time_t*);
 time_t mktime(struct tm*);
+time_t timegm(struct tm*);
 time_t time(time_t*);
 char* ctime(const time_t*);
 void tzset();