LibC: Implement crappy version of pread()

This patch adds a crappy pread() just to get "git" working locally.
A proper version would be implemented in the kernel so that we don't
have to mess with the file descriptor's offset at all.

Libraries/LibC/unistd.cpp

@@ -702,6 +702,16 @@ int unveil(const char* path, const char* permissions)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+ssize_t pread(int fd, void* buf, size_t count, off_t offset)
+{
+    // FIXME: This is not thread safe and should be implemented in the kernel instead.
+    off_t old_offset = lseek(fd, 0, SEEK_CUR);
+    lseek(fd, offset, SEEK_SET);
+    ssize_t nread = read(fd, buf, count);
+    lseek(fd, old_offset, SEEK_SET);
+    return nread;
+}
+
 char* getpass(const char* prompt)
 {
     dbg() << "FIXME: getpass(\"" << prompt << "\")";

Libraries/LibC/unistd.h

@@ -99,6 +99,7 @@ int setgid(gid_t);
 pid_t tcgetpgrp(int fd);
 int tcsetpgrp(int fd, pid_t pgid);
 ssize_t read(int fd, void* buf, size_t count);
+ssize_t pread(int fd, void* buf, size_t count, off_t);
 ssize_t write(int fd, const void* buf, size_t count);
 int close(int fd);
 int chdir(const char* path);