Add closefrom declarations for freebsd, glibc, and, openbsd

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

druntime/src/core/sys/freebsd/unistd.d

@@ -17,3 +17,5 @@ extern(C):
 nothrow:
 
 int getosreldate() pure @trusted;
+
+void closefrom(int);

druntime/src/core/sys/linux/unistd.d

@@ -5,6 +5,7 @@ public import core.sys.posix.unistd;
 version (linux):
 extern(C):
 nothrow:
+@nogc:
 
 // Additional seek constants for sparse file handling
 // from Linux's unistd.h, stdio.h, and linux/fs.h
@@ -21,3 +22,6 @@ char* getpass(const(char)* prompt);
 
 // Exit all threads in a process
 void exit_group(int status);
+
+/// Close all open file descriptors greater or equal to `lowfd`
+void closefrom(int lowfd);

druntime/src/core/sys/openbsd/unistd.d

@@ -19,3 +19,5 @@ int getthrname(pid_t, char*, size_t);
 int pledge(const scope char*, const scope char*);
 int setthrname(pid_t, const scope char*);
 int unveil(const scope char*, const scope char*);
+
+int closefrom(int);