filename -> fname

The rationale for this change is that `makeprgBuild` uses `fname` as the
parameter name to retrieve the file path. Therefore, the documentation
has been changed to match the code.
Merge pull request #1113 from BurntSushi/makeprg-fname-param

update makeprgBuild documentation: filename -> fname
rustc: make arguments configurable.
Remove the rustc checket for Rust.

Between #947, #1114, and #1112, this checker is too controversial to
maintain in syntastic.  Please consider using it as an external checker
if you need it:

https://github.com/scrooloose/syntastic/wiki/Syntax-Checker-Guide#external
Add a note about the removal of the rustc checker.
R lint and svtools: fix CWD for Windows.

On Windows R is run with CWD set to the directory used at install time,
rather than the current directory.  The official workaround for this is
to add links to R from the home directories of each of your projects.
We can't do that in Vim, but we can call `setwd()` to Vim's idea of
current directory.  This mimics the behaviour of R on UNIX.
Add a note about rustc's new home.