Add getpeercred operation on unix request socket

[bjne]

SO_PEERCRED is afaik available on Linux only, and so far only on unix sockets.
This command will not be compiled on other platforms.

syntax:
local client = assert(ngx.req.socket(true))
uid, gid, pid = client:getpeercred()
if not uid then
...
end

[agentzh]

@bjne Should we reuse the existing getoption() method instead? There are just too many getsockopt() possibilities.

Also, I think we should add a feature test for the SO_PEERCRED thing directly in the config file instead of assuming all linux versions do support this thing.

For such things without I/O, it might indeed be more flexible to expose the fd from the cosocket object.

[bjne]

@agentzh
getpeercred is inspired by luasocket's getpeername, but I have no strong opinions here, so just tell me what to do. sock:getoption("peercred")?

Agree with feature testing, will look into it.

so you are suggesting exposing sock:fd()?