vim-patch:7.4.272

Problem:  Using just "$" does not cause an error message.
Solution: Check for empty environment variable name.
          (Christian Brabandt)

https://code.google.com/p/vim/source/detail?r=00228400629e28384f7f52556c3c119ba0d0a44d
Remove incorrect trailing backslash

If this backslash is here and USE_VALGRIND is defined, valgrind complains that "else" is not a command.
revert #652

reverting broad cosmetic/style change because:
- increases merge-conflicts
- increases overhead of merging upstream Vim patches
- reasons for change are ambiguous, so default to no change
Replace 'alloc' with 'xmalloc' in some files.

Files changed: charset.c, buffer.c, diff.c, edit.c,
ex_cmds.c, ex_cmds2.c and ex_docmd.c.

The remaining alloc's in these files require more careful attention to
remove.
Added missing #define guards in headers
Prevent preserve_exit() from executing more than once.

https://github.com/neovim/neovim/issues/563