Fix macOS posix_fallocate().

The fallback ftruncate() call was wrong, because posix_fallocate()
will never shrink a file, but ftruncate() will. Also, if that failed,
we didn't turn errno into a return code.

Also the fcntl() was wrong. In my defense, here's the documentation:

     The position modes (fst_posmode) for the F_PREALLOCATE command indicate
     how to use the offset field.  The modes are as follows:

           F_PEOFPOSMODE   Allocate from the physical end of file.  In this
                           case, fst_length indicates the number of newly
                           allocated bytes desired.

           F_VOLPOSMODE    Allocate from the volume offset.

I think the new version is right, though it's obviously a lot more
conservative than the real posix_fallocate(), but I don't think
it's possible to do better?

Also add tests for some of the prior failures.

Fixes #472.

lib/portability.c

@@ -380,18 +380,19 @@ int mknodat(int dirfd, const char *path, mode_t mode, dev_t dev)
 }
 
 // As of 10.15, macOS offers an fcntl F_PREALLOCATE rather than fallocate()
-// or posix_fallocate() calls.
+// or posix_fallocate() calls. The fcntl only (as the name implies)
+// pre-allocates, so we also need to ftruncate() afterwards.
 int posix_fallocate(int fd, off_t offset, off_t length)
 {
-  int e = errno, result;
+  int e = errno, result = 0;
   fstore_t f;
 
   f.fst_flags = F_ALLOCATEALL;
   f.fst_posmode = F_PEOFPOSMODE;
-  f.fst_offset = offset;
-  f.fst_length = length;
+  f.fst_offset = 0;
+  f.fst_length = offset + length;
   if (fcntl(fd, F_PREALLOCATE, &f) == -1) result = errno;
-  else result = ftruncate(fd, length);
+  else if (ftruncate(fd, maxof(offset+length, fdlength(fd)))) result = errno;
   errno = e;
   return result;
 }

tests/fallocate.test

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+#testing "name" "command" "result" "infile" "stdin"
+
+rm -f foo
+testcmd 'simple' '-l 123 foo && stat -c %s foo' '123\n' '' ''
+testcmd 'shorter' '-l 12 foo && stat -c %s foo' '123\n' '' ''
+testcmd 'longer' '-o 200 -l 12 foo && stat -c %s foo' '212\n' '' ''