This commit adds a new mode option 'e' that must follow any 'b', '+' and/or
'x' options. C11 is clear about the 'x' needing to follow 'b' and/or '+' and
that is what we implement; therefore, require a strict position for 'e' as
well.
For freopen() with a non-NULL path argument and fopen(), the close-on-exec
flag is set iff the 'e' mode option is specified. For freopen() with a NULL
path argument and fdopen(), the close-on-exec flag is turned on if the 'e'
mode option is specified and remains unchanged otherwise.
Although the same behaviour for fopen() can be obtained by open(O_CLOEXEC)
and fdopen(), this needlessly complicates the calling code.
Apart from the ordering requirement, the new option matches glibc.
PR: kern/169320
5ba651f004