Auto merge of #1373 - thibaultdelor:wgetsupport, r=alexcrichton

 Fallbacks to wget if curl not installed

Tested in ubuntu docker with
 1. Both curl and wget installed
 2. only wget installed
 3. only curl installed

Console output for wget looks like this :
```shell
root@845dcc4ba103:/# wget -qO- https://raw.githubusercontent.com/thibaultdelor/rustup.rs/faa08bd786b7282500cc4162a2f428484c891130/rustup-init.sh | sh
info: downloading installer
--2018-03-08 06:54:27--  https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-gnu/rustup-init
Resolving static.rust-lang.org (static.rust-lang.org)... 54.230.135.47, 54.230.135.132, 54.230.135.130, ...
Connecting to static.rust-lang.org (static.rust-lang.org)|54.230.135.47|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 12984456 (12M) [application/x-sharedlib]
Saving to: '/tmp/tmp.DQhaNqhC4q/rustup-init'

/tmp/tmp.DQhaNqhC4q/rustu 100%[==================================>]  12.38M  11.0MB/s    in 1.1s

2018-03-08 06:54:29 (11.0 MB/s) - '/tmp/tmp.DQhaNqhC4q/rustup-init' saved [12984456/12984456]

Welcome to Rust!

This will download and install the official compiler for the Rust programming
language, and its package manager, Cargo.

It will add the cargo, rustc, rustup and other commands to Cargo's bin
directory, located at:

  /root/.cargo/bin

This path will then be added to your PATH environment variable by modifying the
profile file located at:

  /root/.profile

You can uninstall at any time with rustup self uninstall and these changes will
be reverted.

Current installation options:

   default host triple: x86_64-unknown-linux-gnu
     default toolchain: stable
  modify PATH variable: yes

1) Proceed with installation (default)
2) Customize installation
3) Cancel installation

```

Author: bors

rustup-init.sh

@@ -9,8 +9,8 @@
 # option. This file may not be copied, modified, or distributed
 # except according to those terms.
 
-# This is just a little script that can be curled from the internet to
-# install rustup. It just does platform detection, curls the installer
+# This is just a little script that can be downloaded from the internet to
+# install rustup. It just does platform detection, downloads the installer
 # and runs it.
 
 set -u
@@ -41,8 +41,8 @@ EOF
 }
 
 main() {
+    downloader --check
     need_cmd uname
-    need_cmd curl
     need_cmd mktemp
     need_cmd chmod
     need_cmd mkdir
@@ -100,7 +100,7 @@ main() {
     fi
 
     ensure mkdir -p "$_dir"
-    ensure curl -sSfL "$_url" -o "$_file"
+    ensure downloader "$_url" "$_file"
     ensure chmod u+x "$_file"
     if [ ! -x "$_file" ]; then
         printf '%s\n' "Cannot execute $_file (likely because of mounting /tmp as noexec)." 1>&2
@@ -331,11 +331,16 @@ err() {
 }
 
 need_cmd() {
-    if ! command -v "$1" > /dev/null 2>&1
+    if ! check_cmd "$1"
     then err "need '$1' (command not found)"
     fi
 }
 
+check_cmd() {
+    command -v "$1" > /dev/null 2>&1
+    return $?
+}
+
 need_ok() {
     if [ $? != 0 ]; then err "$1"; fi
 }
@@ -359,4 +364,24 @@ ignore() {
     "$@"
 }
 
+# This wraps curl or wget. Try curl first, if not installed,
+# use wget instead.
+downloader() {
+    if check_cmd curl
+    then _dld=curl
+    elif check_cmd wget
+    then _dld=wget
+    else _dld='curl or wget' # to be used in error message of need_cmd
+    fi
+
+    if [ "$1" = --check ]
+    then need_cmd "$_dld"
+    elif [ "$_dld" = curl ]
+    then curl -sSfL "$1" -o "$2"
+    elif [ "$_dld" = wget ]
+    then wget "$1" -O "$2"
+    else err "Unknown downloader"   # should not reach here
+    fi
+}
+
 main "$@" || exit 1