shadowsocks-libev: new package

Shadowsocks-libev is a lightweight secured SOCKS5 proxy for
embedded devices and low-end boxes.

https://github.com/shadowsocks/shadowsocks-libev



Signed-off-by: Min Xu <xuminready@gmail.com>
[Thomas:
 - drop patch adding license file for libipset
 - drop --disable-documentation, it's passed by autotools-package
 - add entry in DEVELOPERS file
 - pass --disable-ssp because the SSP check uses AC_COMPILE_IFELSE()
   so with some toolchains, it think SSP support is available, while
   it's not. And anyway we want SSP to be controlled by Buildroot
   global options, not on a per-package basis.
 - add depends on BR2_USE_MMU, because the code uses fork().]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>