Fix "./util/install.sh -L"

Fix some errors in install script.

My environment:
$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=12.04
DISTRIB_CODENAME=precise
DISTRIB_DESCRIPTION="Ubuntu 12.04.4 LTS"

1: Need curl
$ ./util/install.sh -L
...
Setting up zlib1g-dev (1:1.2.3.4.dfsg-3ubuntu4) ...
Setting up libssl-dev (1.0.1-4ubuntu5.11) ...
Setting up libssl-doc (1.0.1-4ubuntu5.11) ...
Setting up uml-utilities (20070815-1.3ubuntu1) ...
 * Starting User-mode networking switch uml_switch
   ...done.
./util/install.sh: line 778: curl: command not found

2: Need permission (AFAIK, original Mininet can be executed by non-root user)
$ ./util/install.sh -L
...
make is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 9 not upgraded.
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0 24896    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Failed to create the file /usr/local/bin/kerl: Permission denied
 27 24896   27  6922    0     0  23704      0  0:00:01 --:--:--  0:00:01 33765
curl: (23) Failed writing body (0 != 6922)

3: Need permission (2)
$ ./util/install.sh -L
Erlang/OTP R15B02 (R15B02) has been successfully built
mkdir: cannot create directory `/usr/local/lib/R15B02': Permission denied
Destination is not a directory

4: Need setpcap
$ ./util/install.sh -L
Compiled src/linc_us3_queue_sup.erl
Compiled src/linc_us3_queue.erl
Compiled src/linc_us3_flow.erl
==> rel (compile)
==> LINC-Switch (compile)
==> rel (generate)
sudo: setcap: command not found
make: *** [rel] Error 1