MFC r243731,r255303: libc: Allow setting close-on-exec in fopen/freopen/
fdopen.

This commit adds a new mode option 'e'.

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.

PR:		kern/169320