* common/fdlimit: Move fdlimit files to separate package
When go-ethereum is used as a library the calling program need to set
the FD limit.
This commit extract fdlimit files to a separate package so it can be
used outside of go-ethereum.
* common/fdlimit: Remove FdLimit from functions signature
* common/fdlimit: Rename fdlimit functions
* cmd/utils: Add check on hard limit, skip test if below target
* cmd/utils: Cross platform compatible fd limit test
* cmd/utils: Remove syscall.Rlimit in test
* cmd/utils: comment fd utility method
