Change option '-m' to '-M' for consistency of cli

`-m` was added as short option for `--no-mouse`, this is inconsistence
to the rest of the cli since otherwise the short options to disable a
feature are capital letters. Therefore this commit renames the option to
`-M`.

This commit also documents the option in the man page.

htop.1.in

@@ -51,6 +51,9 @@ Show only the processes of a given user
 \fB\-U \-\-no-unicode\fR
 Do not use unicode but ASCII characters for graph meters
 .TP
+\fB\-M \-\-no-mouse\fR
+Disable support of mouse control
+.TP
 \fB\-v \-\-version
 Output version information and exit
 .TP