RLIMIT_NOFILE fallback

In sha1_file.c, when git is built on linux, it will use getrlimit(RLIMIT_NOFILE). I've been deploying git binaries to some unusual systems, like embedded NAS devices, and it seems some with older kernels like 2.6.33 fail with "fatal: cannot get RLIMIT_NOFILE: Bad address".

I could work around this by building git without RLIMIT_NOFILE defined, but perhaps it would make sense to improve the code to fall back to one of the other methods for getting the limit, and/or return the hardcoded 1 as a fallback. This would make git binaries more robust against old/broken/misconfigured kernels.