Userland: Add syscall -l option and man page

Author: https://github.com/mauri870
Commit: https://github.com/SerenityOS/serenity/commit/cb4e51a7a58
Pull-request: https://github.com/SerenityOS/serenity/pull/910
Reviewed-by: https://github.com/bugaevc

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

@@ -0,0 +1,62 @@
+## Name
+
+syscall - test a system call
+
+## Synopsis
+
+```**sh
+$ syscall [-o] [-l] [-h] <syscall-name> <args...> [buf==BUFSIZ buffer]`
+```
+
+## Description
+
+The `syscall` utility can be used to invoke a system call with the given arguments.
+
+## Options
+
+* `-o`: Output the contents of the buffer argument specified as buf to stdout.
+* `-l`: Print a space separated list of all the Serenity system calls and exit. Note that not all the system calls can be invoked using this tool.
+* `-h`: Print a help message and exit.
+
+## Examples
+
+Write a string to standard output:
+
+```sh
+$ syscall write 1 hello 5
+```
+
+Read a string from the standard input into a buffer and output the buffer contents to stdout:
+
+```sh
+$ syscall -o read 0 buf 3
+```
+
+Get the pid of the current running process:
+
+```sh
+$ syscall getpid
+```
+
+Sleep for 3 seconds:
+
+```sh
+$ syscall sleep 3
+```
+
+Create a directory:
+
+```sh
+$ syscall mkdir my-dir 0755
+```
+
+Exit the program with status 2:
+
+```sh
+$ syscall exit 2
+```
+
+## History
+
+This is a direct port of a utility with the same name originated from the Plan 9 operating system. 
+

Userland/syscall.cpp

@@ -29,16 +29,21 @@ int main(int argc, char** argv)
 {
     int oflag;
     int opt;
-    while ((opt = getopt(argc, argv, "oh")) != -1) {
+    while ((opt = getopt(argc, argv, "olh")) != -1) {
         switch (opt) {
         case 'o':
             oflag = 1;
             break;
+        case 'l':
+            for (auto sc : syscall_table) {
+                fprintf(stdout, "%s ", Syscall::to_string(sc));
+            }
+            return EXIT_SUCCESS;
         case 'h':
-            fprintf(stderr, "usage: \tsyscall [-o] entry [args; buf==BUFSIZ buffer]\n");
+            fprintf(stderr, "usage: \tsyscall [-o] [-l] [-h] <syscall-name> <args...> [buf==BUFSIZ buffer]\n");
             fprintf(stderr, "\tsyscall write 1 hello 5\n");
             fprintf(stderr, "\tsyscall -o read 0 buf 5\n");
-            fprintf(stderr, "\tsyscall -o getcwd buf 100\n");
+            fprintf(stderr, "\tsyscall sleep 3\n");
             break;
         default:
             exit(EXIT_FAILURE);
@@ -61,7 +66,7 @@ int main(int argc, char** argv)
                 perror("syscall");
             } else {
                 if (oflag)
-                    printf("%s", buf);
+                    fwrite(buf, 1, sizeof(buf), stdout);
             }
 
             fprintf(stderr, "Syscall return: %d\n", rc);