Implement POSIX lockf.

This has been requested a few times over the years. This is basically
a very late rebase of https://android-review.googlesource.com/45470
which was abandoned years ago. One addition is that this version has
_FILE_OFFSET_BITS=64 support.

POSIX puts this in <unistd.h>. glibc also has it in <fcntl.h>.

Bug: http://b/13077650
Change-Id: I5862b1dc326e326c01ad92438ecc1578d19ba739
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index 90b1db3..1f26a60 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1235,6 +1235,8 @@
     getifaddrs;
     if_freenameindex;
     if_nameindex;
+    lockf;
+    lockf64;
     preadv;
     preadv64;
     prlimit; # arm mips x86