client API: silence silly clang warning

The values compared here happen to be of unsigned enum types - but the
test is not supposed to break if we somehow force the enum to signed, or
if the compiler happens to use a signed type (as far as I remember, the
exact integer type the compiler can use is implementation-defined).
client: remove redundant assignment

This is set by send_reply().
client API: additional documentation comments

(I guess some of the mpv_event contents turned out relatively awkward.)
command: export the flag whether an option was set on commandline

Can be useful for certain scripts; I think someone requested this.