Rename some argument placeholders to be less cryptic,

Rename some argument placeholders to be less cryptic,
in particular s/inl/in_len/ and s/outl/out_len/ as suggested by tb@.

While here, also get rid of the "outm" placeholder that has been
around since the file was added to OpenSSL in 2000, replacing it
with the usual "out" in the four function prototypes affected; tb@
and myself suspect it was simply a typo followed by copy and paste.

Slightly improve variable naming in the examples, too, for clarity and
consistency, even though that doesn't turn the examples into good examples.

OK tb@