Support operating systems that don't have O_CLOEXEC.

Let the caller decide whether to strip.  Only remove \n and \r
(end-of-line markers).

Reduce the number of unnecessary memmove() calls.

Look at the buffer first, before trying to read from the file.  This
reduces overhead because we don't refill the buffer after every line.

Eliminate duplicate code.

The null byte is whitespace, too.

Eliminate the std::string overhead.

This was broken by commit 84d20d9e, which deleted the "!" from the
check.

Revert to the old API before commit e9e55b08, removing unnecessary
bloat.

Eliminate support for partial reads.  The Linux kernel will never
return partial results, so this buffering was unnecessary.

Fixes breakage by commit 1a619522