Set CURLOPT_NOSIGNAL in rest module

This is required for multithreaded applications (see http://curl.haxx.se/libcurl/c/libcurl-tutorial.html#Multi-threading). Once in a while, this would result in a longjump error:

    *** longjmp causes uninitialized stack frame ***: freeradius terminated
    ======= Backtrace: =========
    /lib/i386-linux-gnu/i686/cmov/libc.so.6(__fortify_fail+0x50)[0xf73b16a0]
    /lib/i386-linux-gnu/i686/cmov/libc.so.6(+0xeb60a)[0xf73b160a]
    /usr/lib/i386-linux-gnu/libcurl-gnutls.so.4(+0x8085)[0xf71bf085]

(file and memory locations may vary among platforms).

The crash occured about once or twice per hour while under continuous load, there is no definite way to reproduce it. I haven't seen it anymore with these changes after several hours of testing.